Answer:
The best way to tell what gender a cat is, is by looking underneath the base of its tail. A male kitten that has not been neutered will have two furry testicles right below the anus, and a penis right below the testicles. A neutered male cat or very young kitten will not have visible testicles, but there will be a noticeable space between its anus and penis.
Spayed and intact female kittens alike will have a genital opening (vulva) which is often seen as a tear-drop shape located right below the anus.
It is usually very difficult to accurately tell the sex of a kitten until it is at the very least two weeks old. In males, the testicles do not begin to develop until four to six weeks old.