You know that you are popular if people keep sending you friend requests on Facebook or following you on Twitter. Strange people also keeps saying Hi to you or trying to talk to you. You can also know you are popular depending on the type of friends you hang around with.
Popularity is all in your head. There is no such thing as popular or not popular people. the "popular" people are the ones who think they are, and generally they are bitches who think they're superior to everyone. but don't get me wrong, you can be "popular" by just being really nice to everyone :)