You can tell kittens sex as soon as they are born, before the testicles start to show. If the kitten is a boy, his bottom will look like two dots. One is the anus and one is where his penis hides. On the girls you will see a dot and a slit. I had it described to me once as as looking like a colon or a semi-colon. The boy being a colon and the girl a semi-colon. You can tell this at birth so by 5 months it should be easy for you to tell. If there is a slit, you have a girl. If not, then it's a boy.