|Many times an animal may have worms even though you see no evidence of it. Roundworms (ascarids) are several inches long, look like spaghetti, and may occasionally be seen in the stool or vomit of an infected pet; usually, though, you will not see them.
Hookworms and whipworms are very small and virtually impossible to see in the stool or vomit.
Segments of tapeworms can be seen; they may appear as rectangular segments moving around the anal area of the animal, or as white rice-like or cucumber seed-like segments around the anus.
So basically, except for tapeworms, the best way to diagnose worms in a pet is to have a fecal exam performed by your veterinarian. In a fecal exam, we look for the microscopic eggs of the worms. Eggs may not always be present in the stool, even though a pet has worms. This is why regular deworming is performed even though evidence of worms may not be present. Fecal exams should still be performed regularly to detect the presence of species of parasitic worms, which may not be killed by our usual wormers.