There are multiple ways to know if your pet has worms and most of them are easy to spot. There are also many different kinds of worms and symptoms will vary. The most obvious sign is seeing white worms in your pets stool, for example tape worm can often be seen in dog's stool, or exciting their back end when walking. Round worm in cats is often vomited up. Full grown round worm is about 2 inches in length and looks like cooked spaghetti. There are also some more subtle signs. If your pet is constantly hungry but is losing weight, or if they scoot their bums along the floor. The good news is that most worms as easy to treat.
At one stage I kept a can of carpet cleaning spray handy for those skid marks on the carpet. Now I make a note in my diary for the three monthly administration of the preventative meds. Skid marks are a sure sign.