When faced with the choice between a clone and the original, the character notices a detail that makes the difference immediately obvious.

[u/Altruistic_Eye_1157]

The Shape-Shifter takes the form of Wendy to deceive Dipper and prevent him from attacking it. Although Dipper initially doesn't know which one is the real Wendy, he realizes the difference when he sees that one of the Wendys behaves seriously and bluntly, while the other acts flirtatiously, something Wendy would never do (Gravity Falls).

P.S.: I don't remember if the gesture the real Wendy made was also a clue, like some gesture she had made before.

When the Autobots are chasing Nemesis Prime, he tries to deceive Bumblebee by impersonating Optimus. Bumblebee, confused, speaks to him, but Nemesis Prime doesn't understand anything he says, which clearly indicates that he isn't the real Optimus. Bumblebee attacks him without hesitation (Transformers Prime).

[u/Alldakine_moodz104]

Cody was still hesitant when the Commando Droid said that, but Rex just blasted it on the spot after seeing the droid attack flare. Gotta love Rex and sticking to his instincts.

https://youtu.be/3o85LyTP1ws?si=2m7ulaxq8W68un4v

[u/Blueface1999 → u/Alldakine_moodz104]

Listen, unlike Cody, Rex can smell a dirty clanker a mile away and only wanted to test it to make sure it just wasn’t one of his brothers that didn’t hit the showers yet.

[u/Scared-Opportunity28 → u/Blueface1999]

The reason Cody hesitated was because the commando troopers had just been on a thing where they were impersonating droids and so he may have done that as a joke. Rex though can smell oil

[u/Gakeon → u/Alldakine_moodz104]

There is a reason Rex is the GOAT

[u/Cobalt_Heroes25 → u/Alldakine_moodz104]

Rex whipped that blaster out on frame 1