Lift the kitten’s tail gently
Look at the shape of the openings
Male kittens usually have two round openings with more space between them
Female kittens usually have a round anus and a vertical slit below it
Male kittens often have a larger gap between the anus and genital opening
Female kittens usually have a smaller gap between the anus and genital opening
Male kittens may have a slight bulge where the testicles will develop later
Female kittens do not have that bulge
Compare with a known male or female kitten if possible
Ask a veterinarian if you are unsure