From now on, whenever I am in doubt as to his or her real identity, I'll ask him or her for a high five! Got you there!
Not so fast.
It's also stated:
Transsexualism
A recent study in Germany has found a correlation between digit ratio and male to female transsexualism. Male to female transsexuals (Transwomen) were found to have a higher digit ratio than control males, but one that was comparable to control females.
Now what?
But I don't know why are you so concerned about mistaking a ladyboy to a girl. Usually, when you talk to one up close, you'll already sense it. And if you're still seriously in doubt about their gender, then either ask him/her directly or just look for someone else.