The best way to find out is to make an appointment with your vet. He can confirm that kittens are on the way, and get an idea of how many, by feeling a cat's belly early on, doing an ultrasound in mid pregnancy, or doing an X-ray in late pregnancy.
There are a few clues that you may notice, too.
Her belly will get big around 30 days after she mates. Another symptom that appears as the pregnancy continues, 2 to 3 weeks after she conceives, is her nipples enlarge and redden (also called "pinking up").
Caring for Your Pregnant Queen
It’s rare, but in the earliest stages of pregnancy, your cat may have "morning sickness" that might show up as a lack of appetite or vomiting. If that keeps happening, take her to the vet. With the surge of hormones and changes to her uterus, she may show signs of fatigue. This phase will eventually fade after those first few weeks pass.
Just like many other females in the animal kingdom carrying a bun in the oven (or in this case, about 2-5 buns per litter), your cat may need extra food and calories while she’s expecting.
She’ll eat about 1.5 times her normal diet as her pregnancy draws to a close, so make sure she has constant access to her normal fare. Your vet will probably recommend that you add kitten food or chow for pregnant cats to your pet's meals before birth and during lactation.
Viruses can spread to kittens before they’re born, so keep up with your cat's vaccination schedule. If your pregnant cat is due for her regular vaccination and deworming/flea treatment or needs medication, check with your vet first to make sure the treatment is safe for her. It is best to vaccinate prior to breeding, as most vaccines are not safe to give during pregnancy.