Imho, If the girl has been getting along with you for such a long time, picked up clues that you're interested in her and still haven't revealed to you that she has a bf, the only reason is because SHE IS INTERESTED.
I wouldn't take that as a negative thing. Just slowly move up the Escalation scale. If you go too fast, she may use that as the yellow card on you. But this may not always be the case. It maybe that once you've both gotten to know each other she has to tell you the truth in which then you can use the BF Destroyer.
Before that, there's no need to elicit her into telling you this. In fact, the longer it stays hidden, the more opportunity for you to pretend you don't know about it and game her hard.
Depends how bold you are. If she bluntly tells you, I have a boyfriend you can shove it off with "yeah, I know". If she goes "oh I have to get back to my boyfriend for..." you can say "You have a boyfriend? Wow, he must be like the sweetest guy in the world being there for you and listening to you all the time." This is also known as the straw man method. You can find it here on this forum if you do a search
Good luck.