The flea dirt will look like little black grains. Place some on a white piece of paper. Add a drop of water. The flea dirt will dissolve. If the result is red or dark red, then fleas are present.
Fleas feed on the blood of the host. The flea dirt is actually flea excrement.
You can't just treat the dog. You MUST treat the environment. Look into the life cycle of the flea. The adult is only on the host for a blood meal. The flea spends most of its life not on a host.