You could casually find a way to talk to her, without trying to be too forward. Maybe say hi in the hallways if you pass her, if you spot her alone at lunch or at the mall, maybe go up to her and say something like ‘hey, aren’t you *instert name?* we had class together last semester didn’t we?’ Then you can strike up a conversation about how boring the class was or something.
If she is really shy and does indeed like you, chances are she thinks you have no idea who she is, and will be really surprised that you’re talking to her.
If she doesn’t like you then worst case scenario you might end up avoiding a person you never talked to in the first place. Best case, she likes you back. *shrug*