Because "when you're stupid, you don't know, it's hard just for those around you," I'm going to leave here the five most common signs that can ignite an alarm associated. Of course, they won't use it at all, because they won't find themselves in them, but at least we've gathered them together.
If you look in the mirror and think you're perfect, there's the best chance of being stupid.
If you don't have enough sense of humour that you can self-chaff, it's extremely high chance you're as stupid.
If any joke to you seems to be an inadmissible attack, the odds of being stupid are in overwhelming percentage.
If you feel with all your being that you are always right and nothing manages to convince you that you can be wrong, I regret telling you that we come to the same conclusion: you are stupid.
If you think you are irreplaceable and that the position of the center of the universe changes after the places you bless with the presence, well... I think it's already clear.
These are the five most obvious signs. If you find them in someone around you, you'd better run.