I'm So Confused

This has happened to me several times, including today:
For some reason a conversation will strike up between myself and some girl I don't really know about something in which we are mutually interested. On some occasions, when with friends, they insist that said girl is interested in me (in the boy-girl way). They say that, but I have my doubts and this is why:

How in the hell do you determine when someone is expressing interest in the person, and when they are expressing interest in the subject being discussed? I mean surely not every conversation with a member of the opposite sex with whom one is not well acquainted is an expression of interest. I mean some strange girl might just really want to talk about Kraftwerk or The Clash or Joy Division (Why are all these topics actually bands?) with no ulterior motive. But how can you tell? I wish that they would just say, "Look, I'm faking this, I don't give a damn about The Demics or some song they wrote about New York, just ask for my number, mmkay?" Because some of these conversations are innocent, right? Surely this must happen some of the time. I mean I'm right, aren't I?