Look under the tail
Female cats usually have the anus and vulva close together, with the vulva shaped like a vertical slit
Male cats usually have the anus and penis opening farther apart, with a rounder opening
Male kittens often have a larger gap between the openings than female kittens
Male cats may have visible testicles, especially if not neutered
Neutered males may have smaller or less obvious testicles
Female cats do not have testicles
Adult male cats often have a broader head and thicker neck
Female cats are often smaller and more slender
If unsure, ask a veterinarian to confirm the sex