The Shadow Knows

Deception is a common tool of the trade, used by both villains and sometimes heroes. Shapeshifting and Invisibility are two common ways to disguise one's true nature or intentions.

But no form of disguise is completely, 100% foolproof. If you know what to look for, you can find the truth out. And sometimes, the truth is right behind you. Everything in the light casts a shadow...

And that's the nature of this trope. As a warning to the viewer (or even other characters), a character casts a shadow that hints at their true form or nature. A common variation with characters who can make fake copies or clones of themself or another person/object will have the illusory copies cast no shadow at all to signify that they aren't actually real.

Conversely, sometimes invisible stuff still casts a shadow. This can be Handwaved when the "invisiblity" is actually an illusion rather than actual "not stopping light".

Compare Living Shadow, which occasionally overlaps this trope; and The Mirror Shows Your True Self which is the same with relfections. Sub-Trope of Glamour Failure.

Examples of The Shadow Knows include:

Anime and Manga

In MAR, One of Alvis' opponents has an ARM which clones him infinitely. Alvis figures out where the real one is hiding by the shadow he casts.
In Soul Eater, Death the Kid's shadow is occasionally shown as a skull. It doesn't quite reveal anything the audience hasn't already been told, but generally accompanies a violent reminder that he is a death god.
In Durarara!!, Celty's Cool Bike is really her horse; it still has a horse's shadow.
In the Closing Credits of Freezing, Chiffon's shadow quite accurately reflects her Badass Nickname, The Matchless Smiling Monster.
In the |Black Rock Shooter anime, Yuu has no shadow, which helps set up The Reveal about her in episode 5.

Comic Books

In the Dark Phoenix Saga, the man responsible for Jean's timeslips or rather, the elaborate illusion that makes her think she's having them has a different shadow. When he's revealed as Mastermind, we even get an author's note telling you to go back and look at the shadow in one scene (admit it - you didn't notice either.) It was brilliant at the time, though it loses something by this point due to Late Arrival Spoiler: it's hard to be an X-Men fan and have missed learning Mastermind's real name, which he went by for the first time in this story pre-reveal.

Film

As in this poster for The Omen.
An example of this can be seen in the poster for Star Wars Episode I: The Phantom Menace, which can be seen as picture on the Foreshadowing page.
In the opening scenes of Bram Stokers Dracula Count Dracula's shadow does not mimic his movements, indicating that there's something odd about him.
- Parodied in Dracula: Dead and Loving It, where the Count's shadow likes to act on its own, including humping Mina's shadow. At the beginning, Dracula falls down the stairs. He gets up as if nothing has happened and walks upstairs. His shadow then gets up and limps upstairs, while clutching its back.
In 300, when the Spartans arrive at the destroyed village they are met by a young boy who tells them of the Persian attackers before dying. When he first approaches them, before he is close enough to be clearly visible, his shadow's shape is clearly that of a Persian Immortal.
In The Secret of Kells, when Pangur Ban is in spirit-form as he/she's being sent by Aisling to free Brendan you can see him/her casting the shadow of a cat.

Literature

According to Jorge Luis Borges' Book of Imaginary Beings, the Peryton is a mythological monster said to come from Atlantis, that resembles a stag with the wings and hindparts of an eagle, but casts the shadow of a man.
In I Capture the Castle, when the family first meets Simon, Cassandra notes that from the angle where she stands, his shadow looks like a devil. She references this incident later when she has misgivings about him.
In David Eddings' Sparhawk series (specifically, The Tamuli), the Delphae—AKA The Shining Ones—are capable of using their mastery of light to make themselves perfectly invisible. It's only one of several invisibility-techniques in that world (others involve messing with minds and messing with time), and like all of them, it has a drawback—their shadows are still visible, and can be spotted. It never actually comes up anywhere outside of an initial warning, though. Despite the Delphaic companion of the main cast doing lots of invisible scouting, nobody ever notices her shadow. Ah well.
An especially creepy case appears in Tick Tock. There is one scene where Tommy is running from the monster, and when they pass under a street light, he sees that the creature is casting three distinctly different human shadows. This is creepy enough until you realize that those are the shadows of the three men it just ate.
The One Ring makes its wearer invisible, but in bright light their shadow can still be seen. That's how the goblins almost recapture Bilbo in The Hobbit.
In the Stravaganza series, if you are in a world that is not your own, you will be marked by a lack of shadow. If you see your shadow has returned, your sleeping body in the other world is dead.
In Wheel of Time, Rand, Mat and Perrin are approached by a man named Mordeth who lead them to a treasure trove he's discovered. After his name fails to set alarm bells ringing, it eventually dawns on Rand that something's amiss when he notices that Mordeth casts no shadow.
Similar to the above, it's been stated that "The Dark Casts no Shadow".
This troper recalls a short story entitled The Shadow and the Flash which tells of two rival scientists who both invent invisibility simultaniously by different means, each of which still gives off a subtle hint as to the user's location. One of them, the titular "shadow", invents a paint so black that anything it covers doesn't even register visually... but still casts a shadow.

Magazines

Mad Magazine had a series of features titled "The Shadow Knows" depicting people in a normal setting, while the shadows cast on the wall depict what they (or at least one or more of the people shown) really want to do in that situation, such as beating the crap out of their domineering boss.
- There was also a old Mad Magazine that depicted various B-list superheroes who were parodies of existing ones, one of whom was a Shadow-parody whose only ability was to cloud his own mind.

Live-Action TV

Star Trek: The Next Generation: In the episode "Identity Crisis", a shadow in a video recording from a mission reveals that there was a camouflaged character in the shot.
In the Doctor Who two-part story beginning with "Silence In The Library", the Vashta Nerada are flesh-eating nano-swarms that resemble shadows, and use this fact for camouflage. As such, anyone they're stalking appears to have an extra shadow that lacks a light source.
Supernatural: Castiel does this on purpose to reveal his true nature.
In The X-Files episode "All Souls", Scully is tipped off as to the true nature and intentions of a child protection agent by the fact that his shadow has horns.

Oral Tradition, Folklore, Myths and Legends

An inversion occurs in Classical Mythology: the Peryton, monstruous winged deers, had their shadows as those of men (at least until they killed someone).
Kitsune in Japanese Mythology and folklore retain their fox-shaped shadows even while in human form. Seen here.

Radio

The Trope Namer is of course The Shadow, who happens to be a good example, as his Shadow is still visible when he clouds the minds of his enemies to hide from their sight.

Tabletop Games

While not "shapeshifting" per se, one of the rules of the Mask in Changeling: The Lost is that your shadow always retains features of how you were changed in Arcadia. If you're a mountain goat aspected Beast, your shadow may have horns; if you're an earth-like Elemental, then your shadow may be craggy
This is used in the artwork for the Kitsune in Call of Cthulhu (tabletop game): Secrets of Japan. It's visible form is a humanoid fox, the shadow... not so much.

Theatre

There's a play where a soldier receives a beautiful photograph of his wife and child smiling and waving to him. All he can focus on is the shadow of the photographer on the ground, wondering who he is.

Toys

Anyone using a Mask of Concealment (invisibility) in Bionicle will still cast a shadow.

Video Games

In The Legend of Zelda Twilight Princess, Midna can be seen still on Wolf!Link's back in his shadow, even when she's not there.
- Also in A Link To The Past, when Link uses the Invisibility Cloak, his shadow is still visible.
- And then there's the Aganihm lookalike that appeared as a miniboss in Oracle of Seasons. Three of him appeared at once, but when you lit the torches in the middle of the room only one cast a shadow, and guess which the only vulnerable one was? The attacks from the fakes still hurt though...
- Perhaps played with in the final boss of Links Awakening. The enemy appears only as a shadow, which imitates all the things you'd expect Link to be scared of (several weak enemies, Aganihm, Ganon, and eventually this wierd eyeball spiky wrecking thingy)
In the Pokémon games that take place at Hoenn, you sometimes bump into invisible obstacles on your path.Looking at their reflection in the water over a bridge reveals them to be none less than Kecleon.
In Devil May Cry and Devil May Cry 3, you can unlock an alternate costume of Sparda, the main character's demon father. While he looks like an ordinary (if, very, very pretty) human, the shadow he casts is of his demon form.
In Shin Megami Tensei Nocturne, the boss Ongyo-ki performs a Doppelganger Spin, creating three shadow clones, and hits you hard whenever you attack one of them. How do you beat him? If you fight him during a full moon, only the real one casts a shadow.
In Okami, in order to free a little sparrow from her cage, you must drag an old lady into the light coming out from a hole in the ceiling to reveal her (and her "husband's") true form, evil crow tengu.
- To be more exact, the light is moonlight. Mrs. Cutter (the disguised old lady) says their kind gets too excited at the time of full moon to go out without exposing themselves.
In The Essence Of SaGa Frontier, there's a picture of Red casting a shadow of Alkaiser, with the words "Chang for justice!" beneath it.
In Bayonetta, the titular characters shadow is that of the butterfly demon she is in contract with for the double jump ability.

Web Comics

This Wrong Hands strip combines The Shadow Knows with Foreshadowing.

Web Original

Supervillainess Sweet Synn, from the Global Guardians PBEM Universe, has the power to appear as whatever it is you want in a woman. Her shadow, on the other hand, reveals the batlike wings and tail of the demon she actually is.

Western Animation

The Villain Molecu-Lar in Silverhawks could take any form, but was often given away because his shadow was always of his true form.
One of the stock magical powers in WITCH is an illusion spell called Glamour that can alter one's appearance but not their shadow. This is used to reveal that the Oracle demanding the return of the Heart of Candracar in I is for Illusion is a disguised Nerissa.
In Da Boom Crew one of the Big Bad's lackeys is a shape-shifter that looks any being but she has a shadow that looks like her normal form and it is constantly dancing and making odd gestures. The strange thing is this is not how the heroes recognize her, they only Spot the Imposter because she tried to mimic a person that spoke everything as an Expospeak Gag.
The Avengers Earth's Mightiest Heroes once had Captain America (comics) cast a shadow resembling a giant Skrull.