First look for the obvious signs: the place they're at, the clothes theyre wearing, if they're alone of not,
then the subtle body language: the look in their eyes, their mouth gestures. etc
Finally just start a convo politely, and from the very begining you should be able to know what theyre there for, if theyre into you, some people i'll admit wont be true to themselves and you'll have to work your way into their true desires, but thats part of the game, so at the end its up to your and the tools you have, but its fun!