Honesty does usually work well... but you could go with the entire "what would you do if..." conversation to try and get a handle on her potential reactions. Like, start asking random questions.. what would you do if.. you had to choose between living on the moon and living on mars? etc but keep it moving towards transgendered persons, and toward lesbian.... This is not always neccessarily correct though because a lot of people will say they feel one way but then when it is their own kid react differently.... So your best bet is to just be completely honest about it. An just come out.

But don't come out because you feel you have to, do it because t is what you want and do it whenever you feel comfortable doing it.

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"Penguins. Lovely." - Edward in Eclipse

Eclipse by Stephenie Meyer. You know you want to read it.