I've had a little trouble with the same thing. There's been times where me and my boys hit up a party and one of my guy friends ends up sleeping with the slvt. (It will eventually become inherently obvious as the night progresses, but at first one is never really sure.) And vice cersa. There was times where I got the ONS and my buddies spent hours on a chick and never made it anywhere past a heavy make out session.
The problem with slvtty girls is that they try to hide it, and they usually do a good job of it up until they get clumsy drunk, but usually by that point a guy has already got her in the palm of his hands simply because he showed initiative.
My game has always been to talk to all the woman. The one that is the most reactive to my flirting is usually the girl I will then target most of my attention towards, however it doesn't always work out as planned. I can recall an instance where I was unknowingly talking to the "slvt" but she wasn't giving me as good of vibes as another girl. Simply because she was trying to be covert about things. So I spent all my time with another girl and got nowhere just to find my buddy ****ing the other girl on the side of the house simply because he showed initiative. During my time talking with her, I didn't want to come off as a creep. But my drunken friend, pretty much non verbally propositioned her for sex and got the lay.
Perhaps take a look at how she's dressed? If she looks slvtty, then she probably is, or she is an attention wh0re.
The screening stuff that Nutz posted earlier is also good. Those questions can give you a clue. Who she came with.. where she is staying that night.. etc..