How can I tell if this girl at school likes me?

We have known each other for some time but I think there is more closeness between us now that we are older and in our last year at highschool. We tend to be talking and hanging out together far more that we used to. Chatting on MSN, etc. She always asks what I'm up to and how I am. She even invited me to the senior ball. But how can I tell the difference between good friends, or something more? I'm really introverted and it would be almost impossible for me to make a move or anything like that.