I'm having good luck with being hardcore direct.
What I mean by that is when a girl mentions her bf I just say "cool so when do I get to steal you" she will probably laugh and giggle and boom your intent is clear. So you KNOW and SHE Knows when she texts you it is not entirely innocent. Just be persistent but not chasing, what do I mean by that? She will probably flake a lot and rightfully so, she is IN a relationship. However don't be discouraged just keep living an attractive life that she just needs to be a part of and she will keep getting closer and closer. I'll keep explaining with a real example. First encounter of this girl she interested run through my spin on the MM kino her a lot and get a number close (I DID NOT SHOW DIRECT INTEREST OR COMPLIMENT AT THIS POINT). We talk and I will say one thing about stealing a girl is you are running through a mine field, one mistake and boom you are dead. She hasn't mentioned she has a boyfriend (but she does) flakes on a D2 and I texted her here and there but when a girl flakes that period of her not seeing you causes attraction to die and I was fighting a lost cause. Couple months later go by and I see her again at this local place. This time she is mad at her bf and tells me about it, so I give her my hand and fedora and let her cry on my shoulder and she says "you're such a great friend" TOTALLY KIDDING! What I really said was like "cool so when am I stealing you?" she thought that was too funny and anytime she mentioned anything about her bf I would just say (joking tone) cool mine is bigger and just random 1up stuff totally not being serious. The point here was to not let her beta-ize me. So im like cool just text me when you want to be stolen okay? and she smiled and laugh and I parted ways. That night she texts me and I planned another d2. She flaked on that one though, still didnt care she is just another option, but like I said I am persistent in asking for d2's. I just keep letting her come to me although I do sometimes talk to her but on those times I never asked for a d2 ( I would say 70% of the time she initiates). But I started noticing with each flake I got further. First flake ignored text to see if I could pick her up, second flake was about the same but she talked to me the next day. 3rd flake she called and gave a bs excuse. AND Finally on the 4th flake... well lets say she didnt flake #stolen.

Post note
What I learned is if they don't like their bf very much (attraction wise) they wont say shit and you steal them without knowing.