How do I know which women are in my league?

You'll know by their reaction when you start up a conversation with them. It's not only their facial expression but their body language that will tell you. They are likely to be polite, but if they are not in your league their eye contact will be fleeting rather than resting on your face for more than a couple of seconds.

They won't place their bodies facing yours, or be in contact unless you're in a crowd. They will turn back to their friends or colleagues as soon as possible.