Being a lesbian isn't generally just someone who's had a bad experience with men. She may be bi, she may not. You'll only know if you ask. You don't even need to ask her out. You don't know her very well?
Do you just want to know or are you interested in her? If interested has this also been outside of your dreams? I've dreamed about being with men that horrify me when I wake up!
Can't you ask someone else in the same circle if she's bi? If she's only into women I don't see the point of this being more than a fantasy however you plan to chat her up.