Revealing Injury

Not that kind of revealing injury. Rather this is when somebody makes a revelation due to the fact that the person he is talking to has an injury. The most common case of this is secret identities and/or robbers getting injured while masked, and then the person they fought noticing a similar injury on the unmasked person.

This can also happen in a slightly different way. When a person is injured they have to receive medical treatment, it can reveal things they didn't want to be known. The most common one is a soldier being revealed to be a woman. Or a robot.

Often paired with Cut Himself Shaving, which is the person trying to explain away the odd injury. Even if the explanation is good, it won't fool somebody who knows about the original injury.

Anime and Manga

 * Digimon Adventure 02 has Davis play against Ken in a football match where the latter receives a cut to his leg. Later when the Digimon Emperor is seen in the Digital World Davis recognises the cut. Then Ken reveals that he is the Digimon Emperor.
 * Dragonball Z - Videl points out that the facial scratches Gohan was trying to explain away were pretty much identical to the ones she saw Saiyaman receive when trying to wrangle an escaped dinosaur. She uses this info to blackmail Gohan into teaching her how to fly (She does ask if that's something that can be taught, to her credit).
 * This is sort of borderline, but in Magical Girl Lyrical Nanoha Strikers, late in the season, protagonist Subaru fights some combat cyborgs who are kidnapping her big sister Ginga. She fails to save her and is seriously injured, which is when we see that
 * In A's, Signum declines to take a bath with the others, and after Hayate is out of the room, Zafira correctly guesses that Fate managed to cut through Signum's armor. Signum wanted to prevent Hayate from finding out that they were collecting Linker Cores to save her, as they know that she does not approve of their doing so.

Comic Books

 * The first criminal to deduce Batman's secret identity (a partner of the Joker called Queenie) did so when she spotted the same shaving nick of the jaw of both Batman and Bruce Wayne.
 * Somewhat similar in Batman: The Lost Years, a tie-in comic to Batman the Animated Series, shows Bruce Wayne playing tennis with Barbara Gordon. He comments on how she favors her left side for her defense, which he realizes is the same advice he gave Batgirl the previous night, and thus figures out her Secret Identity.

Film

 * In the first Spider-Man movie, Norman Osborne realizes that Peter is Spider-Man after Aunt May notices that Peter has a gash on his arm, the same gash that Norman (as Green Goblin) had given Spider-Man.
 * A slight variation happened in the third movie, where it was revealed that the Osborne's butler not only knew everything about the Green Goblin project, but had identified that the injuries Norman Osborne had received were clearly from his own glider, thus revealing (to Harry) that Norman had been responsible for his own death.
 * In Batman Returns, Catwoman has to hide the injuries Batman gave her when she's seducing Bruce.
 * In X 2 X Men United, Wolverine recognises a disguised Mystique by the claw-marks he gave her in the previous film.
 * In Aliens, Bishop is revealed to be an android when he accidentally pierces his skin and "bleeds" a white fluid.
 * Subverted in the first Beethoven movie, in which the evil vet fakes an injury after claiming the titular dog attacked him. Later George grabs him by the arm and a moment later realises that there's no injury at all.
 * In Prince of Persia the Sands of Time, Dustan realizes his uncle is the true Big Bad when he notices that his hands are burned from handling the poisoned mantle that killed the Sultan - despite having never touched the robe on screen.
 * Averted in the Green Hornet movie. The titular hero refuses medical attention to the bullet wound in the shoulder he recieved while masked, knowing that it would arouse suspicion if somebody arrived with the same exact injury just after the battle. They decide to fake a drive-by shooting so that he can keep his arm without compromising his identity.

Literature
"Weaver: They do say she [Granny Weatherwax] creeps around the place o' nights, as a hare or a bat or something. Changes her shape and all. Not that I believes a word of it but old Weezen over in Slice told me once he shot a hare in the leg one night and next day she passed him on the lane and said "Ouch" and gave him a right ding across the back of his head."
 * In Harry Potter and The Philosopher's Stone, Harry knows that Snape entered the room with Fluffy because he has a large bite wound on his leg.
 * A variant: In the Leopold story The Four O'Clock Felon, after one crime goes wrong, the villain switches from armed robbery to breaking & entering. The police finally realize
 * In the book Juggernaut, not sure if in the movie, the police are searching for Big Bad as he is making his escape. They are searching for a man with a mustache. A cop is about to let a clean shaven man leave when he notices his upper lip is bleeding. Seems he cut himself shaving.
 * In Star Trek New Frontier, Starfleet figures out that Soleta is half-Romulan after she's badly injured after Jumping On a Grenade for her commanding officer. (How, I don't know, given that her other half is Vulcan, and supposedly they're still close enough to each other to interbreed, so...)
 * There are several tales in folklore of witches terrorizing a town in animal form, until their paw is wounded by a holy man or witch-hunter, who is then able to find the witch when she resumes human form, because her hand will bear the same wound.
 * Echoed in Stephen King's Cycle of the Werewolf. A boy attacked by the werewolf shoots it in the eye with a bottle rocket. Later, he's able to identify the cursed man by the same (unexplained) injury.
 * Also used in Women of the Otherworld. Elena reads a version of the old legend in the Pack's history books: a knight wounded the werewolf's paw, came home to find his own wife bandaging her bloody hand, and killed her. As he gutted her, her unborn wolf cubs fell from her stomach. Overcome with horror, the knight committed suicide. Elena prefers to interpret the belly full of puppies as a metaphor for the knight's guilt when he realized he murdered his wife without letting her explain. In Broken,
 * Parodied in Lords and Ladies:


 * There's a subtle version of something like this at the end of The Wreck Of The Zephyr,
 * The Bloody Chamber uses this in The Werewolf; Little Red Riding Hood takes out the cloth that has a wolf's paw wrapped in it, only for it to fall out as a human hand and reveal her grandmother is the werewolf she fought off earlier.

Live-Action TV

 * In the old Zorro TV serial, Zorro's secret identity was figured out once by the Big Bad of the season. Although there were many Sherlock Holmes-esque clues he cited, they all were just to back up the fact that Don De La Vega had a sword wound in the exact place he had stabbed Zorro.
 * The same thing happens in the movie The Mask of Zorro.
 * In another episode Zorro himself uses this as a Spot the Imposter trick. Facing both the Alcalde and his impersonator at the same time, he conclusively proves which one is the real Alcalde to all present by pointing out that he had wounded the true Alcalde on the wrist some time previous, then slashing the sleeves of his opponents to see which one had a scar there.
 * In an episode of Bones the identity of the Gravedigger is revealed via a rib injury sustained from being hit by a car.
 * The cause of many a Robotic Reveal: Data (while suffering amnesia) in one episode of Star Trek the Next Generation springs to mind.
 * Robin of Sherwood: The new Robin is shot in the leg, a young nobleman starts to limp. People notice.
 * Averted in an episode of Robin Hood. Guy of Gisbornes injure Marian's arm whilst she was in her Night Watchman disguise. Later he casually pats Marian on the arm and notices that she's bleeding. As she's cutting an apple at the time, Marian casually slices her palm with the knife and tells Guy that she accidentally got a little blood on her sleeve. He seems suspicious, but lets it go.
 * In an episode of Dr. Quinn Medicine Woman Sully identifies the leader of the Klu Klux Klan by revealing the burn mark on his arm that he sustained at a rally the night before.
 * In an episode of the old television show It Takes a Thief, Alexander Mundy gets a slash on the arm from the villain's dragon, and later the villain realizes who his opponent is when he studies photographs and spots that the fake skin on Mundy's arm is wrinkling after it gets wet.
 * While not an injury per se, there is an episode of White Collar where Neal gets himself captured and drugged up with an unspecified drug that makes him higher than a kite. When Peter rescues him Neal admits to him that he trusts Peter more than anyone - a revelation he probably wouldn't have made if he weren't so loopy.
 * He also admits to stealing some manuscripts via carrier pigeons while in this state.
 * In the first episode of Wild Boys, Hogan has a pistol shot out his hand. The troopers later identify Hogan as the bushranger because of the injury to his hand.

Video Games

 * In Tales of Symphonia when takes an attack at the end of the first disk, it damages her sleeve enough to reveal that . The scene plays out rather odd if you are in a new game+ and using a costume title, as they default to the normal costume mid cutscene.
 * In the fourth case of Phoenix Wright Ace Attorney, . Invoked in the third case, when someone steals the Steel Samurai case and pretends to be limping from a sprained ankle to cast suspicion on Will Powers, who sprained his ankle.
 * In Apollo Justice Ace Attorney, the fact that Alita Tiala's scarf hides turns out to be a crucial point of evidence.

Web Original

 * In Sailor Nothing, Aki is only able to recognize Himei in her Sailor guise because she has a black eye that Aki had noticed earlier in the day.
 * Homestuck: Karkat has been hiding the color of his blood for his entire life, so the readers don't learn it until Jack stabs him. In-universe, it's a subversion, though- while Karkat starts freaking out, no one else there actually cares.

Western Animation

 * In Mulan. the heroine is revealed to be a woman when she saves her commanding officer from death and is injured in the process. Their physician has to patch her up. He tries to be discreet about it, but informs her commanding officer, who is forced to expell her from the army. (Technically he's supposed to kill her, but he still felt gratitude so he let her live.)

Real Life

 * Real Life: Deborah Sampson enlisted as a man in the Continental Army during the American Revolution. She served for a year and a half, including several battles. Deborah came down with malignant fever and was cared for by a doctor, Barnabas Binney. He removed her clothes to treat her and discovered her secret. Dr. Binney later told George Washington, who never uttered a word and honorably discharged Deborah Sampson from the Army at West Point.