oooh, Greg's answer rocked.
Ok. Seriously though. I am a BIIIIG flirt, there are several men that have taken it to mean I was interested in them outside of just being a bit flirtatous (I am a bartender -it's kinda in my job description), and have taken my attentions to mean that I was interested beyond the initial interaction.
Best bet? Ask her straight out- "Hey, is this just friendly or what, because I'd like to get my groove on, and if it ain't you, I gotta mozy looking for that elsewhere" - if she answers like she's just friendly, don't dump her as your friend- girls are valuable assets for getting you into the sack with other girls! Just accept and maybe move on to the next conquest
Best of luck!