With boyfriends, you basically need to figure out if her bf is the problem if its your approach.
One way to measure is that even girls who have boyfriends will give you their number if you are cool. This is how I usually get it...
Her: I have a boyfriend
Me: Fo real! Dayum, congratulations. You in love?
Me: You got kids?
Me: That's two strikes. How long you been with him?
Her: Blabla years.
Me: You gonna marry him or what?
Me: Fair nuff. So here's the deal: I am gonna message you in 6 weeks exactly to see if you are still with him. If you are not, we're grabbing coffee. If you are, well, I hope you fall in love and make some babies.
If a girl buys into this, then I think its a good indicator that your vibe is cool and you have some attraction. If a girls who throw you bfs don't buy into the above, it's a good indicator that your approach, not the bf may be the problem.