My index finger is considerably longer than my ring finger. 2-4 digit ratio is a good indicator of prenatal androgen exposure, or lack thereof.
There are a number of other indicators as well, such as the rate at which one builds muscle mass, a certain face shape, being able to cross your legs twice, overall flexibility, penis size at birth, and a few more things.
Genetic females with the opposite issue tend to be stronger and more aggressive.
Either way there is a very high rate of transexuality. I don't recall the exact figure.