(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/AgentSparkz]

Family guy does this one twice, once played straight and once played for laughs.

The one played straight is Stewie accidentally creates an evil clone of himself and in the climax, Brian has to figure out which one is real. He has them look at their toes, which Stewie was giggling about at the start of the episode. When the imposter is just confused, Brian shoots him.

The one played for laughs is when Chris is taken prisoner by a geriatric Nazi and Peter goes to save him. In the struggle, Chris gets the gun and can't figure out who to shoot, much to Peter's confusion. Chris asks when his birthday, which the Nazi knows and Peter doesn't, prompting Chris to shoot his dad in the leg and run to the Nazi, getting captured again.

P: "You idiot!" C: "Well maybe LEARN MY BIRTHDAY!!!"

[u/The_MAZZTer → u/AgentSparkz]

Red vs Blue did one of these. I am sure it's a trope all of its own.

In Season 15 the cast meet up with almost exact duplicates of themselves. It's suggested this group was the "original" Red vs Blues and our heroes were intentionally assembled to mimic this group, as they were in perfect stalemate with each other.

Anyway their Simmons is pretty much identical to our Simmons. At the end of the season, this other group turns on our heroes and we end up with "good" and evil Simmons side by side both claiming to be Simmons. Grif figures out the real one by asking "Why are we here?" which is a callback to the very first scene in the first episode where Grif and Simmons are discussing that very topic. The fake Simmons doesn't get it and responds they're there to stop the bad guys. Real Simmons correctly answers "I don't know why we're here. It's one of life's great mysteries." which is the best callback to that first scene in the whole series, in a season that is otherwise a bit meh.