It's difficult to tell the sex of a newborn kitten. It's generally recommended to wait until it's about a month old before checking.
To distinguish between male and female cats, you can look at the shape of their anus and genitals. Female cats have an "i"-shaped anus and genitals, a short distance between the anus and genitals, and are almost hairless; conversely, male cats have a ":"-shaped anus and genitals, a wider distance between the anus and genitals, and some hair.
