No One Else Is That Dumb

Yugo and Aki: Who are you?
Keiji: It's me, Tokiwa, yo!
Yugo: What's 1+1?
Keiji: 3!

Yugo: It really is Tokiwa!

—Silver Spoon

In some cases where the party has been split or someone has changed so much you hardly recongnize him or her, you may want to make sure the Bob in front of you is the person you know, and make some kind of check; for example by asking him something only the real Bob could know. But what to do when Bob is usually so stupid he doesn't really know much? Well, you will check just that instead. Ask a question expecting an answer so silly that only the real Bob would be capable of pulling it off. Of course, Bob may also show his idiocy without waiting for any question. Inversely, giving an unusually intelligent answer will reveal the imposter.

Compare Trust Password and Bluff the Imposter. Will often overlap with Spot the Imposter.

Sub-Trope of Something Only They Would Say. See also Who Would Be Stupid Enough...?.

Examples of No One Else Is That Dumb include:

Anime & Manga

The above quote from Silver Spoon is after Keiji made a dubious relooking during his summer vacation.
In the early volumes of Naruto, anticipating a separation and knowing they are eavesdropped, Sasuke deliberately makes up a long, complicated password as a check. When Naruto comes back after they split up, he says the password flawlessly... which identifies him as an impostor since the real Naruto would have forgotten such a long phrase immediately.

Live Action TV

Blackadder II: the Master of Disguise Prince Ludwig has infiltrated the court of Queen Elizabeth; the palace is having a fancy dress party to take her mind off her troubles and Blackadder (who was Ludwig's prisoner) told Ludwig that Nursie always dresses as a cow. Then Blackadder escapes, returns to London, and stabs Nursie while she's dressed as a cow - but it isn't Nursie, it's Prince Ludwig.

Edmund: This was the information with which we bought our lives. We told him that, if the Queen was having a party, Nursie always goes as a cow. From that moment, he was doomed. All we had to do was escape, return, and kill the cow.
Queen: How could you be sure it was not Nursie?
Edmund: Because, My Lady, Ludwig was a master of disguise, whereas Nursie is a sad, insane old woman with an udder fixation. All we had to do was kill the one that looked like a cow. That was the mistake I knew that Ludwig would make. His disguise was too good.

Western Animation

There was an episode of Banana Man where the protagonist was distinguished from his fabricated clone that way.
The episode of Family Guy where Brian has to distinguish between the real Stewie and his evil clone - both are naked and there are no other clues. Brian eventually hits on the fact that the real Stewie will revert to type and behave not as a world-dominating evil genius but as an eighteen-month old toddler. He asks both to look at their feet. The evil clone sneers while the real Stewie is captivated by his own wiggling toes. Brian then knows which one to shoot.
In an episode of Goof Troop, Goofy encounters a criminal who looks just like him. In order to tell them apart, Max sets down a bucket of wallpaper paste and asks his real dad to please step forward. Max immediately knows that the one who clumsily steps in the bucket is Goofy.
In an episode of Inspector Gadget, a criminal Master of Disguise infiltrates an important police meeting as Gadget himself. When the real one shows up, he accidentally knocks down chief Quimby with his mallet, prompting Quimby to order the other one arrested.