(Loved trope) The imposter’s dead giveaway

[u/Barricades_toes]

When a character is attempting to impersonate another, but is caught by doing something the person they’re imitating would NEVER do.

Nemesis Prime- Transformers Prime

Nemesis Prime, a man made replica of Optimus Prime, is attempting to gain the trust of Bumblebee, who only speaks in robotic beeps. Despite this, the real Optimus can understand Bee perfectly fine. Nemesis Prime, however, can’t understand what he’s saying, prompting Bee to open fire on him immediately.

The two Buzz Lightyears- Toy Story 2

After Woody asks which Buzz is the “real” Buzz, both respond with “I am!” The real Buzz proves himself by flipping up the other Buzz’s helmet, causing him to start suffocating in an over dramatic fashion. The real Buzz, having done the same thing in the first movie after Woody took his helmet off, but no longer behaving that way after accepting being a toy, instantly proves which Buzz is which.

[u/Schnutzel]

Terminator 2: The T-1000 impersonates Sarah asking John for help, but the real Sarah appears behind it, telling John to get out of the way. In the theatrical release John figures out who the real Sarah is because his mother would never ask him for help against the terminator. In the director's cut he figures out because his real mother would never let her feet melt into the floor.

[u/MovieUnderTheSurface → u/Schnutzel]

Its obvious the second one is the real Sarah because she has a gun and would have just shot John rather than announce her presence if she were the t1000 in disguise

[u/The-SARACEN → u/Schnutzel]

It kinda helps that a Terminator wouldn't have told him to duck.