To define a sex of a kitten is much easier, if newborns are both males and females, as you can compare one kitten with another. The problem is that the cats have a very similar genitourinary system.
If you hold the kitten in the hand so, that its bag is on your palm, you can see, first, the fundament (the outlet opening of a rectum), then the genitourinary opening. For females this opening represents a vertical slot arranged very close to a fundament. For the male two small testicles can be lower than fundament and, directly, under them you can see the small round opening of a prepuce. However, the testicles can be invisible for the first twelve weeks after the birth.
It is the only way to define a sex of a kitten. As a rule, it is customary that red cats - are males, and tortoise - are females, but there is no rule without exception.
It is easier to define the sex of a kitten immediately after the birth or after the first month of its life. But if you have a fluffy cat it is better to define its sex immediately after the birth, before its wool will start to grow.