The whole transgender discussion on UKP always fascinates me.
There are a
lot of self-proclaimed experts on here.
The
only way to tell since the advancement of modern science is:
1. Ask her (If that doesn't work, go to Step 2)
2. Get a saliva sample while she's asleep
3. Throw it in the mail to Ancestry.com or 23andme.com

Although that's probably a little intrusive.