While fleas and lice may look the same to the naked eye, they are in actuality very different insects. Other than the fact that both fleas and lice are small and wingless, they differ greatly. Fleas belong to the order Siphonaptera, and lice are of the order Phthiraptera. While fleas and some lice thrive on blood and living skin, certain species of lice prefer dead tissue instead.
Note how much the insect is moving around. Both fleas and lice jump around, but fleas are much more active than lice.
Look at the insect under a microscope. Fleas are flat and have extra-long legs; their hind legs in particular are very long. A lice's legs are more stout and they have claws and their bodies are oval, not flat. This is because lice have to hold onto their host and stay put. The flea will be dark red or brown, while a lice will be dark grey.
Verify where the insect is living. If you find the insect living on an animal, particularly under the arms or behind your pet's ears, it's most likely a flea. Lice are most often found on humans, particularly in their hair.
Find the insect's eggs. Fleas and lice both lay oval-shaped eggs, but flea eggs are white and lice eggs are a shade of tan. However, if the lice eggs are hatched they'll be white at this point as well. Lice eggs, or nits, are difficult to spot on the hair shaft. You may mistake nits for dandruff.
The main symptom of lice is that your scalp will be itchy and irritated. Pets who are infected with fleas will itch as well. If you think a human has been bitten by fleas, check around his ankles and feet for raised, irritated lumps.
While lice don't typically carry diseases, fleas not only carry disease but can spread them as well. Lice can be spread from person to person. Everybody is susceptible to head lice regardless of personal hygiene. Speciality shampoos and conditioners designed to kill head lice should be used if you're infected.