Look under the tail
Female cats usually have the anus and vulva very close together
Female cats usually have a vertical slit below the anus
Male cats usually have more space between the anus and the genital opening
Male cats usually have a round opening below the anus
Male cats may have visible testicles if they are not neutered
Female cats do not have testicles
Compare the shape of the openings
Check for a larger gap in males and a smaller gap in females
If unsure, ask a veterinarian to confirm