How do you know if your crush likes you back?

I am nice to girls who like me but for some reason it doesn't seem to be the other way around, like girls that know my friends like them start to hate them for some reason. I think he likes you back though, go for it. Please answer: a href="http://uk.answers.yahoo.com/question/index;_ylt=AjsPNggnM5DHhTy1hHvUgBgx.;_ylv=3?qid=20090626134800AASM8rX"http://uk.answers.yahoo.com/question/ind…/a