Determining the sex of a kitten is actually not too difficult. All it calls for is a little patience and some delicate handling.
Wait until the kitten is a month old.
It is nearly impossible for a layman to correctly guess the gender of a kitten below the age of 3 weeks.
Firstly, handling these newborns for long isn't even recommended, because you'll only end up passing your 'human' scent onto the kitten, making it hard for the mother to identify her baby.
Worst case scenario, the mother may end up abandoning the kitten.
There are a few humans equipped with the ability to remain unfazed by the allure of tiny, tiny kittens. It doesn't matter if those furballs grow up to be pioneers of what we call a resting bitchface, they can still melt the coldest heart with their infinite cuteness.
When you bring a kitten home (from a shelter or a rescued one, hopefully), the first thing you would want to do is to give it a decent name. No, there's no need to compromise by picking a unisex name; there are ways to determine your kitten's gender at home. Pay close attention to what follows.
How to Tell the Sex of a Kitten
Before we begin, note that the kitten in question should be at least over the age of three weeks for you to venture a fairly accurate guess.
First step, ensure that the kitty is calm, and not in a playful mode whatsoever. Even a young kitty knows how to protect her privacy, and doesn't mind using her (very) sharp claws. Kittens sleep a lot, so a good time would be when it begins feeling drowsy.
Hold the kitten very gingerly, as the precious little thing is still very delicate. Now, stroke it lightly to calm it further. You need to take a look at the kitten's posterior to find the answer. Therefore, the tail needs to be moved. Do so very gently. If the kitty becomes agitated, set her down lightly, and try this later.
If not, you need to take a look just below the tail; this is where you'll find the anal opening. This looks the same in case of both, males and females.
The next and most vital step is to look lower, where you'll find either possibilities:
In case your kitten is female, you will spot a vertical slit (the vulva) very close and right below the anus.
In case of male cats, the genitals are further down, about half an inch lower from the anus, and have a rounded shape.
And you're pretty much done.
Other (Unreliable) Methods
While the one mentioned here is a sure-fire way to find out the gender of a kitten, there are other methods which involve a considerable amount of guesswork, and are thus not too reliable.
Calico, or tortoiseshell, is a pattern in cats, indicated by a tricolored fur (white, orange, and black). This pattern is caused by the genes linked to the X chromosome. Therefore, the resultant kitten is almost certainly a female.
The calico pattern is found in several breeds, and it is extremely rare that one comes across a calico-patterned male.
It is often found that most orange tabby cats are males; although the results aren't too definitive.
In older, intact cats (those who have not been spayed or neutered), the males tend to have chubbier cheeks―look for a fuller jaw along the whiskers. Females have a more angled, tapering face.
If you are still unable to determine the sex of your kitten, just take it to the vet who is sure to answer your queries.
Also, regardless of its gender, it is your responsibility to nurture this little pet into a grown, healthy cat.