How to tell
If she uses you as a shoulder to cry on, constantly complaining about her other relationships...you're totally in the "friend zone" and won't ever be tappin that.
If she finds reasons to hang out with you even when you don't have to ask her to, and she does subtle touches to you (like brushes your arm with her hand when she laughs, or touches your leg to get your attention) ... she totally digs you and wants to go out with you.
If she brushes her hair back from her ears and crosses her arms sort of slouchy and insecure and lets you "hit on her" in teasing fashion ... it may be hit or miss. Give her some compliments, and see where things go. Or, give her some backhanded compliments to run her self-esteem down more, and then make her feel like no other guy will date her so she might as well date you. All depends on if you're a good guy or an a-hole.
You don't have to be a nice guy to be a good man. Have some confidence. Chicks love confidence.