I think things like this. Whether I'm interested in the person or not, I wonder, "if I say something, will she be annoyed and think I'm hitting on her, or would she actually want me to say something; maybe it would really make her day, and make her feel better about herself." It used to be (because I only heard girls complain about guys hitting on them) I'd be afraid of the former, and not say anything. Now days though, I feel the latter is probably more likely, and even if not, it's worth the risk to try.