Question about trans-gender children?

I have a q, how do doctors tell babies are hermaphrodites when they have one genders part down stairs and the opposite genders part upstairs cuz all babies chests look the same, so how could they know before the child hit puberty. I saw a show a year ago about a baby who was a few months old that had manly parts upstairs but womanly parts downstairs and they were trying to decide if it should be a girl or a boy but I was just like how do you tell?