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.
Bigotry
It isn’t necessarily about intelligence, since stupid is as stupid does.
I tend to call that willful ignorance. It’s knowing that there are other ways of thinking, but refusing to even consider them. Which is stupid, dumber than dammit, and/or moronic. I’ve known some damn intelligent people that are stupid by virtue of not using their brain.
Be aware though, not everyone can control their volume. I don’t doubt you were aware of that, but it’s not so rare a thing as to not be worth mentioning. There’s also others that have trouble controlling volume.
Again, I don’t doubt you meant the folks that can control their volume, and do so without difficulty, just adding to things.