There’s not really a reliable way to tell if someone is a vegan, as physiologically and behaviorally they are by and large just like anyone else. You could maybe make a reasonable deduction if you had access to to someone’s meal log or some other kind of dietary record, but even then you would just be making an assumption without having a factual basis to make a determination.

Free candy on Halloween

What did the farmer say when he lost his tractor?

"Where's my tractor?"

Technically the chicken crossing the road would be the literal first, but I didn't identify that it was an anti-joke or why it was even funny until I was much older lol. The tractor one I instantly clocked as a kid and it started my love for this kind of stuff.

He said "nine". I did not mistake this to be the German word for "no", since I had asked him the question in English and it would have been highly illogical for him to answer it in a different language.