How do I tell if a girl likes me as more than a friend (I'm a girl too)?

I'm 15. I've always been straight, but suddenly I've fallen for her. I really like her, and hope she returns my feelings, but I don't know how to tell between just friendly interaction and deeper feeling.
She's a real tomboy. She wears almost exclusively men's clothes, acts like one, and from afar it's hard to say if she's a girl or a boy. I asked a mutual friend about her sexual orientation - apparently she's straight.
I'm pretty sure she has something going on in terms of gender, though.

Our friendship so far is kind of on the surface. We talk mainly about mutual interests and share jokes.
She hasn't made many moves to interact with me outside of a small groups of friends she sits with (I join her, not vice versa), but I can tell she enjoys my company. When she looks at me, she looks me right in the eyes. She's complimented them, and remembers every single detail of our conversations, which courses I have and when, with who, who my friends are, it's kind of scary. When we were doing the 'school wide run', she stuck by me although she could have gone with other friends. She laughs very easily at my jokes, or any of my comments/behaviours. She jumped at the chance to help me with things, too.

How do I know if she's just being a good friend and wants to appear friendly, or if she likes me back?

Thanks for any answers.

Well, you could talk about bisexual and other relationships. Like asking "There's so much going on in the gay community, what's your view?" To see where she stands. Then maybe hang out one on one? And start becoming close friends and see what happens. If the time is right then tell her how you feel.