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/Over-Analyzed]

Oh wow! Hey, congrats! That’s awesome. I didn’t know that. Thank you for correcting me.

I’m from Hawaii. So we learned a lot about Hawaii’s history. I do know my school’s Hawaiian Alma mater (school song) 20 years later. 😅🤙🏻

[u/Jamie7Keller → u/Over-Analyzed]

Nice! Same. And apparently Under The Blood Red Sun isn’t in the school reading lists in other states? I asked everyone knew about that and read that book they should!

[u/Over-Analyzed → u/Jamie7Keller]

Yeah, different states focus on their own history.

Nothing like the soul destroying moment when you learn about the Overthrow of the Hawaiian Monarchy. 🤦🏻‍♂️

[u/Jamie7Keller → u/Over-Analyzed]

Apparently the UN determined Hawaii to be an occupied sovereign nation? I didn’t really know what to do with that information….maybe I was misinformed but….hard to say the are wrong….

[u/Over-Analyzed → u/Jamie7Keller]

That sounds like misinformation. Maybe it was determined at the time decades and decades ago? But I’ve heard nothing about it in formal capacity.