It depends on its age, as it is really difficult to tell when they are kittens.
Basically if he is a "whole" (unneutered) tom, it should be obvious. You'll see his testicles. Even at 4-6 weeks you should be able to feel (very very gently) 2 little peas under the skin. If your cat is older than 6 months, he may have been neutered, but look closely and you may see 2 very small remnants of the sac.
Otherwise, it's probably a female. Some people comment that the female looks like the letter "i" from behind, with the split being the vagina and the dot the anus.