One of the best tools available for detecting fleas is the flea comb. This is a small, metal-toothed comb. The teeth of the flea comb are very, very close together. Fleas are flat little critters, and if the teeth were not close together they would slip between. A flea comb allows you to comb through the hair and pick up bits of flea dirt and the fleas themselves.
Often the first sign of fleas that you see is the excrement they leave behind, rather than the fleas themselves. If you part the hair on your pet, particularly in areas such as the base of the tail and the back of the neck, you may see small specks of "flea dirt". We call it flea dirt because it is hard to distinguish from plain old dirt. On the photo to the left, all of the black flecks are flea feces. The blue arrow is pointing to a flea who is "fleeing" into the hair. We were lucky to catch him in the shot!
The photo at the right shows some flea dirt that we combed from the tail base of a cat with a few fleas. You can tell that these are flea feces because several of the bits are curved or comma-shaped. It would be odd for actual dirt to be this shape.
Another way to determine whether this is "just dirt" or "flea dirt" is to add water. Flea dirt is flea excrement. Yep, flea feces. Flea poop, if you will. Since fleas only eat blood, it makes sense that their feces are made of digested blood. If you put flea dirt on a white tissue, add a few drops of water, and wait for a minute the digested blood will start to dissolve and you will see red streaks (see photo to left). If it's regular dirt you will just make a little mud, and it will stay black or brown.
If your pet has a large flea load, you may see fleas as well as their feces in your combings. Over on the right is a picture of a flea running around in the hair and flea feces.
You can distinguish fleas from lice by the speed of movement. Fleas are very quick, and tend to hop. Lice are slightly smaller and move very slowly. If you stare at them for a while you can see them moving around and it's obvious they are alive, but fleas positively zoom.