@speedy827,
Well, let's see. You could try asking her out and if she says no, then she doesn't like you. Of course, if she says yes does not necessarily mean she likes you, only that she wants an evening out. In that case you may need to actually ask her if she likes you. Or maybe the next time you are near her you could lean over and plant a kiss smack dab on her lips and see whether she pushes you away or kisses back.
So pick one. Any of these should work, although I will warn you from experience that the kiss thing could also lead to a slap on the face. But if you don't mind that risk, then the reward is definately worth it. Unless she hasn't brushed her teeth in a week. Then it might not be worth it. Unless of course you are really desperate for a kiss, in which case it may still be worth it.
Uh, what was the question again. Oh yeah, I remember. Just ask her. That's the quickest way to be sure.