Determining a cat's gender is important whether you are sexing kittens, strays or a cat you wish to adopt. Feline gender identification is quite easy once you know what clues to look for.
Handle cats carefully and gently. Wait until kittens are 3 to 4 weeks of age, if feasible, before trying to hold them -- make sure that the mother cat is comfortable with you handling them, and be ready to return the kitten to the mother if either becomes distressed.
To handle wiggly or ornery adult cats, enlist another person's help. Encourage the tail up by petting or scratching the area where the tail meets the back.
Determine gender by measuring the distance between the anus and genitals. Do not touch the genital area but rather eyeball the spacing. Female genitals appear like a vertical slit and are either directly below or within a half-inch below the anus. If the distance is greater than a half-inch in a kitten or an inch in an adult, the cat's a male. Male genitals appear more rounded. This scrotal sac is not fully developed until about 6 to 10 weeks of age. Neutered adult male cats' genitals appear smaller than those of male cats who aren't neutered.
Call the Vet
If you're not sure, take the feline to a veterinarian. The vet should be able show you the male or female genitals and determine whether the cat has been fixed. Rarely, a kitten is born without an anus -- a life-threatening birth defect. If you notice only one opening, take the kitten to the vet immediately.