As I recall from the lecture by Robert Sapolsky, researcher can look at a preserved brain and determine with a fairly good accuracy if the brain came from a woman or a man. Experiments has then show that for transgender people the same is true for the gender the brains person identified as, including for people who has not undergone any hormone treatment.

I would have to rewatch those lectures to see if he gave any direct examples of markers, numbers, or sample size.