Normally, cat guardians have suspicions (or know for certain depending on their level of knowledge) that their cat has tapeworms when they see tapeworm body segments containing eggs in their (1) cat’s faeces or (2) vomit or (3) on the fur surrounding their cat’s anus. The tapeworm body segments are called proglottids. When they are fresh and moist they move. They are about 0.25 inches (6.3 mm) long. When they are dry they resemble grains of rice attached to the fur.
Occasionally, a tapeworm releases itself from inside the cat’s intestines and moves to the cat’s stomach. If this happens the cat may vomit an adult tapeworm which might be several inches long.
Another rare way that a cat guardian might have suspicions that their cat has tapeworms is if they debilitate their cat or cause weight loss. This would happen if they are present in large numbers.
Finally, also rarely, a cat might scoot or drag its bottom across the ground or carpet because it is irritated by the presence of proglottids. Apparently, this form of behaviour is more common in dogs than cats.
Tapeworms are the most common intestinal parasite in adult cats. They vary in length from less than 1 inch (25 mm) to several feet in length (3 feet is about 1 meter). The head of the worm fixes itself to the wall of the small intestine using suckers and hooks on its head (scolex). The worm is made up of segments and each segment contains eggs.
As I understand it, deworming pills result in the cat digesting the worm because it is unable to prevent it. The cat flea is part of a tapeworm’s lifecycle.
Some more on worms