<p>This is a solution for you figure out what she thought about you, but this might adversely affect your friendship. Ignore the fact that she has a boyfriend, in fact, act as if you don't know that. And ask her out. And, it's important for you to conjure the fact that you don't know. </p>
<p>So, maybe start the conversation like: </p>
<p>"Remember how last time you asked me if I would like a girlfriend... well I thought about it, I was too shy then, but I think it'd be really cool for us to be together. Just for fun."</p>
<p>Best outcome: If she really like you, then she dumps her current boyfriend.
Good outcome: She tells you no because she has a boyfriend, but tells you have she did like you before.
Average outcome: She tells you no, and she tells you that she didn't like you.
Bad outcome: average outcome + weakening of friendship. (too awkward)</p>