A male cat has fairly noticeable testicles underneath his tail when they are newborns. The female may not show anything, but perhaps you might be able to see her vaginal orifice, underneath the anus.
If your newborn cat does not show any signs of round protruding organ below its tail, then it is certainly a female. you can tell because a boy has a boys thing and a girl has a girl thing
The male's part is further away from the anus, where the girls part is closer.