How do you know if a girl likes you and what you should do if she does?

The answer is yes. Now how much she likes you is an answer that only she can answer. If you like her, just stay calm and be approachable to her but do not give up doing thing you like to do just to be by the phone if she calls. Do not tell her you like her if you do. You can show her by hanging out with her and chatting with her and invite her some place. The suspense and the process of finding out if a boy like a girl is more interesting to a girl then you just saying I like you. The longer you can keep her interested the better your chances are in making this long term. The best way to keep her interest is to give up things about yourself slowly, ask her things and let her talk.