Almost all (but not ALL) kittens of calico (black, white and orange) or tortoiseshell (black and orange) are females. The opening just below the tail is the anus. A male has a much greater distance between the genitals and anus than females, which are ½ inch apart for a male kitten or more than 1 inch apart for a male adult cat. In kittens of similar size, the distance between the anus and the genital opening is greater in the male than in the female.
A female kitten’s genitals are closer to the anus and look like a straight line or a tear.