Carnes right just ask her outright, tell her what her friend said and ask her to be honest with you, and have a crafty look for tell tale signs such as an Adam's apple a more pronounced jaw line I'm sure you would know if she had been a man just by looking at her a man is a man at the end of the day wether his bits have been chopped off or not, it's beyond me why anyone would want to be a woman it's a hard life you know.
you'd be surprised theres a reason why they feel the need to change their gender and you could HARDLY tell sometimes if they're transgender or not, and thats because they're ment to be the other sex then what their born
as for the story, you need to have a calm sit down talk, this is a big issue, this is like not telling someone you were married before, or have children , big deals you need to talk about before you go any further.
Love is love, most of us preach that love is love now , and we should actually take that into consideration in these situations, weather or not she was male at one point she obviously NEVER considered herself male, to her she was always a she,
dont leave someone you could truly have a good life with because people will make fun of you, or call you names, dont let people bully or judge you, if you truly want to be with her, then do so.
and obviously youre not gay she has a tits and a vagina, thats not gay lol