If, based on symptoms, you suspect that your cat has worms, you should visit a veterinarian. Take your cat and his stool and/or vomit samples (place them in a zipper bag) with you. Most signs mentioned here can also signal other, more serious medical conditions, and applying worm medicine in an inappropriate situation may do harm to your cat’s health.
Note that your cat won’t have all of the worm symptoms mentioned above. It’s likely that a cat with even severe worm infestation will display only few or no signs from the list. This is because there are different species of internal parasites and that cats can develop resistance to worms, showing no apparent symptoms.