Is this chick simply not interested in me? How do I know for sure?

In my college psych cl, there is this one chick I met through one of my friend's (also a chick). Both of us are 20, we major in the same thing, and we were born 5 days apart. Typically, I talk to her before and after cl a little bit about random things. I have her friended on fb but when I send her IMs through fb chat, she usually doesn't respond. Is this chick simply not interested or is she playing around? How do I know for sure? Is there anything I can do to find out if she's interested or playing hard to get? I was thinking about asking her out by the end of this week, but her actions are making me unsure. Any advice?