So from the novel...
Spoilers
In around i think chapter 779 he gets the documentation at the hospital in the country side in which he(she) was born in because her stepmother wanted to confirmed if she was actually a girl or sum. So he gets the documentation and he opens them and it shows that she was born a female. He gets surprised surprised . He then goes to the room because she was sleeping on his bed. And he checks under her clothes carefully to confirm and yep. That’s it. And I think he goes to the bathroom after that
https://www.lightnovelworld.com/novel/national-school-prince-is-a-girl-09011258#:~:text=National%20School%20Prince%20Is%20A%20Girl%20novel%20is%20a%20popular,other%20chapters%20are%20in%20progress.
idk if this is still valid tho but u can try read here !
could someone tell me how long until he finds out that he is actually a she?