Yeah, most people can't narrow it down that close unless they are very familiar with said populations, but I think pretty much everyone can, as you say, tell when someone's white or black or something.
Yeah, that's definitely one. I personally didn't really suspect I was genderqueer until college (although looking back I can sure see the signs in retrospect) but several of my friends insists that all transpeople are born that way and always know from early childhood.