May I just say I'm going to find out some private information about my girlfriend from her friends?
Really? Why do you want to ask your friends private information about your girlfriend?
I want to know what she's not telling me about herself.
It's understandable to know everything you can about your girlfriend, but she may have things she hasn't shared with you yet because she's trying to protect her privacy. You shouldn't try to pry that information from her, when she's ready she'll tell you. Respect your girlfriend's privacy.
I'm going to keep asking.
Privacy is something sacred to many. If you pry too much these friends of hers will go to her and tell her what you've been asking. Or they might give you the wrong info on purpose to spite you, so be careful not to pry like that.