A trained eye isn't usually needed to spot a heavy flea infestation. You'll probably notice your pet scratching, biting its coat, or showing other signs of discomfort. If you then run a fine metal comb through your pet's coat, you might see them crawling around on the comb afterwards.
Often though, you'll notice symptoms of a flea infestation without being able to find the culprit. That's because pets, especially cats, will groom fleas out of their coats long before you've had a chance to try and find out what's making them itch.
If you're in any doubt, carry out a simple flea check. First sit your cat or dog on a large piece of white paper. Then rub its back vigorously for a minute or so. As you rub, any flea faeces will fall onto the paper. You may need to hold the animal's tail between its legs in order to prevent it moving whilst you do this. Next pick up the piece of paper, remove any hair, and transfer the 'rubbings' onto some damp cotton wool. Leave to stand for a minute.
Flea faeces are made up of dried blood from the host they have bitten. When dry, they are dark brown flecks that can be easily confused with dirt or dead skin. But once transferred onto the moist cotton wool, they'll dissolve and turn a lighter shade of red. So, if you can now see red spots on the cotton wool, you can be certain that your pet has been in recent contact with fleas. Treatment is required.