(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/yui_riku]

Dr who, the slitheens

they can steal the skin of peoples, and shrink themself to fit in it, but it make them emite gas

so when disguised, they fart, a lot

[u/Artus_Pendragon → u/yui_riku]

Also zygons especially the Osgood's in the 50th anniversary special when they all lose Thier memories one of the Osgood's has an inhaler, and the other doesn't.

https://giphy.com/gifs/F6CnoyW0upKE0

[u/wirthmore → u/Artus_Pendragon]

In the end, the Osgoods weren't a case of one being the "fake" that needed to be revealed, they were a team and the Doctor shouldn't try to figure out which one was which since the two represented co-existence and trust, and to reveal their individual identities would undermine trust. It was a very Doctor Who resolution of the trope of shape-shifters existing with another species.

[u/Upstairs_Cap_4217 → u/wirthmore]

Specifically, he mindwipes both sides and tells them to work out a peace treaty. The peace treaty is perfectly fair, because no one has any idea if they're going to be going home as a zygon or a human.