Dark copper coloured and about the size of the head of a pin, fleas dislike light so looking for them within furry areas and on the pet's belly and inner thighs will provide your best chances of spotting them.
Look for "flea dirt," too. "Flea dirt" looks like dark specks of pepper scattered on the skin surface. If you see flea dirt, which is actually flea feces and is composed of digested blood, pick some off the pet and place on a wet paper towel. If after a few minutes the tiny specks spread out like a small blood stain, it's definitely flea dirt and your pet has fleas!