*coming from a straight male's perspective here *

I've always had this problem in bars/clubs/pubs etc when you go out and you catch someone looking at you and obviously you go over and intro. yourself

But...I've had many times found out that the person looking at you is either

a. seeing someone
b. has a bf already
c. is lesbian

this is AFTER you've already bought them a drink and spent what 10-20 minutes talking to them. By the end of a night you might have talked to 3-4 women and i can honestly say about 1/2 of'em were either A,B,C or even D (they are engaged/married and didn't have their ring on lol) and quite frankly its wasting your time (unless you're after an affair which i'm not looking for)

So with that in mind I've been looking for ways of identifying either A,B,C or D earlier but in a "silver-lining" type of way without being so direct and perhaps prior to "Can I buy you a drink?"

What can you come up with?