Usually you can tell if a girl is flirting with you by the way she acts. First you need to make sure you are not getting the wrong message. Usually a girl is flirting with you if they laugh at alot of things you say, being touchy with you alot, do some things so you two can be together alot, ect.
If you see that she is really flirting with you, you dont have to go crazy just be cool about it. Girls like it when boys are laid back.If you are laid back you dont need to do it too much because it might show that you are not that interested in her.All you have to do is be you self and flirt as far as she wants to go(but dont push it).Then one day just jokingly ask r u flirting with me and see what response she makes.Or you can just be honest and tell her that you like flirting with her.