This is a problem I can fully agree with... Personally, I think people should have icons to carry around like IM status icons that say "Interested", "Friendly", "Do not approach", etc! lol! (It would make this whole "are they available, are they interested, etc..." thing a whole lot easier!

My personal gauge is how much interaction with that person they initiate, as well as how open they are about things beyond casual conversation... Eg: did she IM you first to say "Hey, how's it goin", did she keep on talking about her home & life & troubles & such, when normal conversation would've dried up before that..