Adults must feed on blood to reproduce, which is why they spend their lives on cats and dogs. Treat the pet, the house and the yard. Only by attacking the flea on all three fronts can you be successful and win. Simple measures taken early will make it unnecessary to mount massive campaigns later in the season. Don't expect fleas to attack only the route you have chosen to defend.
Fleas are masters of camouflage and sneak attacks.
Look through the fur down to the skin. Black specks that look sort of like pepper are flea manure. If you're not sure whether the specks are manure, wet them with a small drop of water. Manure will dissolve into what looks like, and is, blood. Since fleas are such a universal problem, there are many products for fighting them, but many of these products are not effective.