This girl keeps looking at me, im not sure if she's interested or not, what should I do?

I've seen her do this through the corner of my eye and sometime when I face her, she looks right at me, this has been gong on for ages. It's kind of awkward for me because I'm not sure what she wants. Occasionally she will talk to me and be really nice and friendly but other times she can be quite rude and obnoxious( she may whisper something to her friend's ear and laugh). I'm really shy and so I never look her in the face, I'm not sure what to make of it, how can I tell if she's interested or not?