It can be difficult to tell if your cat has worms (unless you see the evidence yourself). Kittens with worms may have diarrhea, slow weight gain and a potbelly. Infected adult cats may have dark tarry stools, vomiting, diarrhea and weight loss. However, both are just as likely to have no signs of their worm infection at all. If your cat has never been checked for worms, it's an absolute must. You should also have a stool sample checked for any new cat brought into your home. Deworming is usually a standard part of the protocol for shelter cats, and young kittens are typically dewormed several times as they get worms from their mother's milk and will pass them back and forth. Even if your cat has been treated for worms before, a reinfestation is possible at any time because treatment just kills the existing worms.
This is one case where you have to go to the veterinarian for help. She needs to diagnose the type (or types) of worm infesting your cat in order to prescribe the correct medication. Over-the-counter deworming medications often don't have enough punch to knock out worms for good. No home or folk remedies have been shown to be both effective and safe enough to get the job done, either. An infestation that goes unchecked for months or even years robs your cat of vital nutrients. She will also be shedding eggs or spores and infecting other animals (and could even infect you).
Cats who go outdoors, hunt, eat raw or undercooked meat or meat products, have fleas or share quarters with a cat who has been diagnosed with worms have the highest risk of being infected and should have a stool sample checked by a veterinarian. In the case of worms, prevention is the best cure. This includes regular flea treatments. The good news is that most common worms usually aren't dangerous, although untreated cases -- especially in cats who are already ill -- can be. So get your cat to the vet and rid him or her of those nasty parasites; you'll both be happier for it!