Just as a general rule, I avoid girls who are all by themselves, unless it's daygame at a mall or something. Girls by themselves during nightgame usually mean that they:
1. Don't have a boyfriend. There could be something wrong with Girl #1 who can't seem to get a guy.
2. Don't even have any girl friends to hang out with. Is Girl #2 really that lonely a person that she can't have her own social circle? How many girls that you know who were worth a chance didn't have ANY friends?
3. She just had some sort of bad experience. Maybe she just broke up with a boyfriend recently, got fired from work, had someone just yell at her, etc. Girl #3 is worth gaming, but not tonight. She's just not in the mood. She came to the bar to get out for a bit and relax. She's alone because she doesn't want any of her friends to have to suffer through her bad attitude/venting period.
If you judge she's Girl #3, which honestly it is, most of the time, you can ask her if there's anything wrong, and if you don't mind sitting through some sob story for few minutes, she will usually feel pretty connected to you, so you can just build some attraction right after and BAM.
If she's still cold after you asking if there's something wrong, eject. She's not going to be up for anything tonight.