How can I tell the sex of kittens that were just born?

A male's genitalia is further from the anus than a female's . The shape of a female kitten's vulva looks like a slit. The penal opening of the male looks like a tiny circle. The kitten's coat color can also indicate sex. Calicos are female, rarely male. Red cats are usually male, but in this case, coat color can be deceiving as females can be red too.