Mine is the volume of their voice. If all that they can do is just talk too loudly with no reason for it and don’t control the volume of their voice. That to me is a sign that you’re a stupid fuck. Because people think the louder they are, the more righteous and attention seeking they are. Than it is to just listen, digest and respond accordingly to anyone talking around you.
Thinking about it, a lot of things that I flag as idiotic come from putting emotions first. We’re all emotional creatures, but idiots are ruled by them. It’s idiotic to blow your rent money on candy, but the short term emotional high for an idiot is too appealing. It’s stupid to blow up at someone instead of admitting fault, but admitting fault feels bad, so the idiot can’t take that. Reading an essay is informative, but that’s boring work and the idiot might feel bad if they don’t understand it, so they’d avoid the whole thing. If the essay doesn’t agree with their worldview, that’s going to feel terrible.