I’m going to skip the whole moral argument that you shouldn’t hit on a girl who has a boyfriend. If you don’t believe in hitting on girls who have boyfriend then that’s cool, otherwise read on.
First, don’t even bother with newly engaged or married girls. I think you’ll have better chances with a nun than a girl who just made a huge commitment with another man. It takes at least a few months for that to wear out.
That leaves two other types of girls with boyfriends: ones that are unhappy and ones that are neither unhappy nor quite ecstatic either. The best way to find out what type of girl you are dealing with is the length of time it takes until she drops the boyfriend.
The longer amount of time until she drops the bomb, the better. It’s best if she doesn’t mention the boyfriend at all and you learn from a third party. Think about that for a second… if a girl is happy with her man, he will keep popping up in her head, leading to a large amount of guilt when talking to a player she is attracted to. She attempts to relieve this guilt by mentioning him as much as possible in the conversation.
The sooner she mentions him and the more times she mentions him, the less likely you will get anywhere. This should be your guide to see if you should commit to gaming a girl with a boyfriend, especially since they will take more work than a single girl.