How do you know you are a girl?

If you can't determine it for yourself, ask your parents or doctor.

If this is a serious question, then yes asking your parents or doctor can help, but in the end, you define what you are; if inside you like what girls around you like, if you don't like what boys like, and it's not because you like a few things, then you are likely a girl. We're talking about traditional actions and behaviors. What's between someone's legs doesn't define what sex they are, that's between the ears.
^^^ Just wanted to say, that's a really good statement. I like it a lot.