My Kepler Bb humanoids are externally gender neutral until they are 5 years old. So you can only tell if they are male or female before then via genetic testing or an abdominal scan like an ultrasound, X ray, or CT.
Now here is my idea as to how males and females develop in childhood:
At birth there is only a tiny nub. Females have this too so this isn't a sign. The testes haven't descended from the abdomen at this point. There is no need to since sperm production doesn't start until the male is around 15 years old. Because the nub is so tiny, there isn't the characteristic spray of urine. It just flows out like it does in females. At 5 years old a special hormone is secreted. It is the childhood version of testosterone. This triggers the nub and the urethra within it to grow longer but not wider(at least not wider than it would normally grow at this age). It also triggers the testes to descend and the skin in the groin to form a pouch. Once all that is done, childhood testosterone production plummets down close to 0. When adolescence nears, childhood testosterone production completely stops and then in adolescence, adult testosterone is produced and this triggers sperm production and erections and makes it easier to build muscle.
Like males, females have a tiny nub. There is also skin and muscle over what would otherwise be the vaginal opening. At 5 years old childhood estrogen and progesterone is secreted and this triggers the skin and muscle over the vagina to die off. Any bleeding there stops and the skin right next to it forms 2 sets of folds, in other words 2 labia(minor and major respectively). Childhood estrogen and progesterone plummets until near adolescence it is at 0. In adolescence, adult estrogen and progesterone is produced and this triggers breast development and the menstrual cycle. It also makes it easier to gain fat and harder to gain muscle.
This production of special childhood versions of sex hormones that only triggers external genitalia formation, I think is the most plausible way for the child to go from externally gender neutral from conception to definitive male or female at 5 years.
But is this plausible having 2 different versions of the sex hormones, one that triggers external genitalia formation(childhood sex hormones) and one that leads to sexual maturity and is produced until death(adult sex hormones)?