Im a girl, how do I know if a girl likes me?

im a girl and I like a girl at my school, I know she's gay so thats good. but the only thing that is weird is that when I've tried to add her on Facebook she won't accept me! she ignores me...really weird, because she acts like she like me in real life..:/
please help! thanks

Say to her at school, "Hey, I didn't mean to bother you when I tried to add you to my Facebook." Then she'll explain.