When is a girl in love?

For starters, you seem young to me. Just that one sentence tells me that you have a sort of immature outlook on the subject. If that's the case, you have plenty of time to contemplate these sorts of things and you will find that the question answers itself when you find the right person to share those deep-down heavy emotions with. There should be no question about how you feel about EACH-OTHER. If you're concerned that you've got someone who's "in love" with you and you don't like her and want to create some distance, don't worry. Being in love is not a one-way street. Someone can have a crush on you but being in love is something that takes two. Be nice but don't encourage her behavior toward you or lead her to believe that something might happen down the line.