Sometimes other people really do know us better than we know ourselves. When I came out to friends as lesbian, they all said yeah we knew that. (They were all very supportive of my coming out.) Of course they didn't tell me 'you are such a lesbian' even though I am sure they were tempted. It would have been completely counter productive.
OP, good luck! If she is anything like me, be prepared - she might freak out a bit. But telling her and letting what happens happen is your best option.