How to Check if You're an Introvert?

Recently I was so happy, when few people called me an introvert. And it happened, because I refused their offers to take a part in some events, including a TV show. I guess I've always been such a person. But, I wonder how to check if you're an introvert without such offers? I mean, what if someone wants to know whether or not he's an introvert, what can he do? How to be sure in it?