Gonna share some knowledge about a little flirting move I use all the time called the "Look Back". This move is gonna work best with someone you've already approached and built some rapport with. Let's say you're talking to this person at the bar, you're joking around, bantering, flirting, etc. You won't want to give them too much attention, so on purpose you end the conversation by saying "Hey, I'll be back in a minute...I'm gonna go say hi to a friend of mine over there". Say it casually so shes not put off. Walk over to your group of friends, making sure shes still within the line of sight and close of enough to still see you. Say a few words to your friends, act like you're into the conversation...and then pause. Innocently turn around and look back at her, with a very subtle smile. If you did you're job in part 1 (flirting and bantering) then she'll still be looking in your direction a lot. Make sure your eyes catch. When they do, she'll think that even though you're talking to you're friends, you're still thinking about her. Do it a few more times before going back to her and you're in.

On the flip side, you might notice someone doing this to YOU...also a good sign that they're flirting with you and probably interested.