You don't need to know.
Leave this alone... and you're so called "sources"... these aren't 100%.
The only people who know for sure about her "lesbian past" is your girlfriend and whoever was with.
Now flip this situation on yourself.
How much would you enjoy your girlfriend badgering you about ANY relationship you've had in the past?
I HATE talking about past relationships. I always steer clear of these conversations as it adds NOTHING to the current relationship. Eventually a fact or two about it might trickle down, but only because people feel comfortable enough to share such details.
I know this for a fact through a variety of sources. How can I bring this up with her and find out what the story is?
There's NO way you can bring this up and not seem like an insecure little boy. She WILL get defensive and I'd guarantee that her first response to you will be a look of shock and then:
"Where'd you hear that from? Who told you that?"
Then after that, she'll look at you like you were some sort of stalker and be disgusted about digging into her past.
And then, you'll probably be single shortly after, because she can't trust you enough. She'll always be wondering what other things you are digging up about her and what people might be telling you.
Why do you need to know anyway? What benefit do you gain from finding out?
Don't tell me you've told her about every person you've gone out with. Have you? I hope not.
So leave her past in the past and worry about the PRESENT.