Everyone is smart.
Sure, most of us aren't winning Nobel prizes left and right, but no matter how much schooling you've had, or what your grades looked like, you are smart. You might not excel in mathematics or history or chemistry, but there is something that you're very knowledgeable about (even if it's a very small something).
There's one thing that most of the smartest people in the world have in common: They don't go around bragging about how smart they are. It's in pretty poor taste and doesn't exactly make people respect or admire you.
Despite that, there are still people in this world who won't shut up about how smart they are. Most likely, they took some kind of online IQ test and can't wait to tell you that they got a super high score on it. These people are kind of the worst.