Uhhhh... no menstruation? He/She didn't pee blood once a month? Or was there some "lost in the middle" kind of condition where there is a uterus but no periodic ovulation?
I mean, this is just a really freak exception to the rule. Any blood work done on him during one of many possible common hospital tests would've shown that this person is a woman. Must be someone who has lived a secluded life and... well, I know I probably would if I was so equipped.