Parents Know Their Children

A parent-child bond is strong enough that even in situations where the child is in disguise, is being impersonated by someone else, or is in the midst of a crowd of look-alikes, the parent can still identify him or her.

Commonly associated with mothers due to the maternal bond that's usually created through pregnancy, childbirth and the early years of child-rearing. However, this phenomenon isn't limited only to mothers, as fathers and other relatives can qualify for this as well.

Compare Something Only They Would Say, which is where a character who's in disguise or whose identity is uncertain makes himself known by saying or doing something that only they would normally do. May also overlap with Spot the Imposter in some cases, but it is NOT the same trope.

Also not to be confused with That Thing Is Not My Child, where a person denies parental responsibility to a child because said child was born via cloning or other unnatural means.

Often the parental/familial method of beating a Twin Switch, especially when the Identical Twin ID Tag isn't in effect.

An inversion of this trope would be the child being able to identify his/her parents despite the specified conditions.

NOTE: To qualify for this trope, the persons MUST be related, whether by blood, adoption, or some other family-related clause. True Companions don't count, as while they may have the semblance of family ties, in the strictest sense of the word they are not family and generally haven't raised the child/children from birth or infancy.

Examples of Parents Know Their Children include:

Anime and Manga

In the Tenchi Muyo OVA, while Yosho's false persona of Katsuhito Masaki had been known fact for a while, his mother sees right though his disguise of being OLD (which was part of that persona), and asks him to take it off. No-one else knows/knew.
- Also in the movie Tenchi Muyo in Love, Achika recognizes Tenchi as her son, even though she is in high school, unmarried, not dating, and the fact that he won't be born for another nine years.
In Hunter X Hunter Reina died and got reincarnated as a chimera ant and her mom recognized her.

Film

In Jett Jackson: The Movie, this is how Miz Coretta, Jett's grandmother, figured out that Jett had switched places with his TV role, Silverstone. When Silverstone asks her how she knew, she says that from the moment she first looked into the then-newborn Jett's eyes, she knew she would always be able to identify him. This information later becomes important in defeating the shape-shifting Big Bad in Silverstone's world when he makes himself look identical to both Jetts.
In the remake of The Parent Trap, the father looks each twin in the eye and declares which one is Hallie. We never find out if he's right though, since the twins keep playing up the charade and make him question his own judgment.
Tangled: Implied to be the reason Rapunzel's mother recognizes her in the end despite her trademark long blonde hair chopped and brown by the end of the film.
Inverted in Spirited Away, where the protagonist's parents have been transformed into pigs; to rescue them, she must pick them out of a line-up of several dozen other pigs. She correctly determines that none of the pigs are her parents.
Stardust: Una and Yvaine are captured by the witches. Tristan, trying to rescue Yvaine, mistakes Una for one of the captors and is about to attack her when she identifies herself, having recognized him even though the last time she saw him was when he was an infant.
Changeling, a 2008 movie based on true events, is kick-started when a missing boy is returned to his mother—but she immediately realizes it's an imposter. She is committed to a mental institution when she refuses to acknowledge the boy as hers. It's a ploy by the kidnapping case's lead investigator to cover up the police department's incompetence and corruption.

Literature

Finn Family Moomintroll has a case of this, where everyone is playing hide and seek, and Moomin decides to hide under a hat which nobody knows to be an artifact which randomly shapeshifts whatever is inside. Naturally, nobody recognizes him afterwards. In the end, he turns to his mother for help, she takes a look and says "Yes, that's Moomin". This also reverses the magic.
Subverted in Tree Castle Island by Jean Craighead George: after Jack discovers on a camping trip that he has a long-lost twin named Jake, and the two return to civilization, Jack's mother (adoptive mother, never knew he had a twin until now) insists that she recognizes the boy she's raised. She goes and stands in front of Jake, not realizing that the two had switched where they were standing when she wasn't watching.
Heckedy Peg, a children's picture-story book, provides a variation: an evil witch turns seven children into seven different types of food. Their mother tries to rescue them, but the witch says she will only change them back if she can guess which child is which food item. She guesses right.

Live Action TV

CSI episode "Split Decision"—identical quadruplets are involved in a killing conspiracy, even though one was a lot younger due to the in vitro fertilization aspect of the story. The mom of one of the four said she'd know him anywhere, but didn't know the other quadruplet who'd been ID'ed at the time.
In Fringe, during Season Four, although Peter is both out of his original timeline (making him unknown to everyone in the current one) and in the other universe, his mother Elizabeth still recognizes him as her son grown up.
Played with in Angel in which Illyria disguises herself as Fred in order to trick Fred's parents into believing that their daughter is still alive and well. They seem to fall for it... though as she's about to leave, Fred's mother wonders if there's something a bit "off" about her daughter, before shrugging it off.

Video Games

Inverted in Devil May Cry, where Dante could recognize Trish (who has his mother's face) even when she was disguised as Gloria.
In King's Quest VI Heir Today Gone Tomorrow, if Cassima's parents are brought Back from the Dead, they can identify that the Cassima who is so eager to marry the Vizier is definitely not their daughter.

Western Animation

In the Family Guy DVD movie Stewie: The Untold Story, Future Lois is able to recognize her infant son when the rest of the family isn't.
In the She-Ra pilot, it's established that Princess Adora was kidnapped as an infant. When Prince Adam brings the now-adult Adora into the throne room, King Randor and Queen Marlena immediately recognize her.
On The Cleveland Show, when Roberta is made to disguise herself as a fat girl as part of a school project, no one at school recognizes her and Junior ends up falling in love with her. When she agrees to go on a pity date with him and he introduces her to the rest of the family, they all somehow recognize her instantly.

Real Life

In many instances in the animal kingdom, a mother knows her offspring by their scent. The inverse is true as well, with animal babies identifying their mother the same way.
Another example, which also appears under Flags of our Fathers: The mother of one the Marines who raised the famed flag over Iwo Jima recognized her son in the picture - from the back. She knew her own child's back and buttocks well enough that she insisted it was him even though officially, it wasn't. Eventually, she turned out to be right.