It isn't always easy to tell whether a cat or kitten is a boy or girl, especially with young kittens whose genitals are not yet fully formed, but, on un castrated males, you can often feel two bumps which are the testicles starting to form.
One common rule-of-thumb used is that if you lift up the tail and see one hole, then it’s a male, two holes, and you’ve got a girl cat.
It can be difficult because a castrated male won’t have any testicles, also, although the cat’s sexual organs are the same as humans, dogs, or any other mammal, they are small and hard to make out.
One certain method - ask someone who’s an expert, like your vet, or someone who’s kept cats for a long time.
