Look under the tail
Females usually have the anus and vulva close together
Females usually have a vertical slit below the anus
Males usually have more space between the anus and genital opening
Males usually have a round genital opening below the anus
Males may have a visible scrotum between the anus and penis opening
Neutered males may have a smaller or less obvious scrotum
Compare the distance and shape of the openings rather than color or size alone
If the cat is very young, sex can be harder to tell
A veterinarian can confirm the sex if needed