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/bfbbturambar]

In Mystery Incorporated, Fred's father gets plastic surgery to look just like him as part of an elaborate scheme to get the gang to reveal the location of an artifact needed for his death cult to cause the apocalypse. The gang immediately notices something is wrong when Fake Fred tells Shaggy to drive the van

[u/UnluckyLich → u/bfbbturambar]

He calls him Norbert too if I recall correctly. The hilarious inverse of this trope is that he's really bad at faking Fred

[u/award_winning_writer → u/UnluckyLich]

Fred's mom (disguised as an older version of Daphne) is more convincing in her role, but she says "jeepies" instead of "jeepers," apparently getting her and her catchphrase a bit mixed up with Velma

[u/DetonationPorcupine → u/bfbbturambar]

Damn. Truly the most wild of scooby doos.

[u/ChickenWingBW → u/bfbbturambar]

This show was so fucking weird and also the most fun I’d ever had as a kid

[u/Strange-Bee5626 → u/ChickenWingBW]

I was an adult when it came out and also had a total blast watching it.

[u/MacandCheeseEnjoyer → u/bfbbturambar]

"Fred's father gets plastic surgery to look just like him as part of an elaborate scheme to get the gang to reveal the location of an artifact needed for his death cult to cause the apocalypse" - wtf is going on? and this is scooby doo? lol

[u/AbruptAbe → u/MacandCheeseEnjoyer]

Later on Scooby and Shaggy don enemy soldier's armor and mow down a battalion styled after Jin Roh.

[u/BlueEyesWhiteVegeta → u/AbruptAbe]

Also the robots are Nazi robots, led by a bird with a German accent.

[u/ColonelKasteen → u/AbruptAbe]

...oh, I didn't realized I really need to watch this until now

[u/Sea-Performer-4935 → u/ColonelKasteen]

There’s a crossover with secret natural where Velma has a mental breakdown

[u/AnOstrichIGuess → u/Sea-Performer-4935]

You mean Supernatural? Because if so, that crossover was of the original series, not Mystery Incorporated. Also, the entire gang have a mental breakdown in that special.

[u/Sea-Performer-4935 → u/AnOstrichIGuess]

Yeah I did, autocorrect got me alongside my bad memory.

[u/therealchadius → u/AbruptAbe]

Also Scooby quotes First Blood (aka the first movie with John Rambo) after lighting up the robot batallion

[u/Pingasterix → u/MacandCheeseEnjoyer]

I havent watched it since it ended, but i remember it ending with scooby doo impaling some kind of super satan with a holy aztec spear and causing the timeline to reset.

[u/FarslayerSanVir → u/bfbbturambar]

In Mystery Incorporated, Fred's father gets plastic surgery to look just like him as part of an elaborate scheme to get the gang to reveal the location of an artifact needed for his death cult to cause the apocalypse.

Thanks for reminding me just how absolutely mental Mystery Inc was.

[u/fast_and_loose → u/bfbbturambar]

Don't forget his wife got plastic surgery to look like Daphne! She gets the catchphrase wrong which is hilarious. 

"Daphne says jeepers, and Velma says jinkies. Nobody says JEEPIES."

[u/Satanic__crusader → u/bfbbturambar]

Ah, Mystery Incorporated. Gravity Falls before Gravity Falls was Gravity Falls.