Testing the waters is really quite easy. Testing the waters about homosexuality is the same thing; generally peope who are gay-friendly are trans-friendly. If you live in California, bring up prop 8, ask what she thinks about gay marriage, etc. I guess that can work even if you don't live in Cali, it's just more relevant here I suppose. Another good idea is to research "holidays" regarding trans/gay issues, such as umm Matthew Shepard's deathday, or trans remembrance day, or PRIDE parades in SanFran or I've heard they have them in other states as well. Bring that up, see how they react.
If they react as gay/trans-friendly, it's all good. If they don't, I don't agree with Nero's approach to just blurt it out. Even if someone reacts negatively at first, they may get used to the idea over time and turn out to be really supportive. Just try a gentle discussion about whatever issue you used to test the waters, put out your own beliefs about gay/transgenders, if you're brave you can even try a "well if I told you I was transgender, would you hate me?" and go with that.
Best of luck.