How can I tell if the black specks in my cat’s fur are flea dirt?

Comb a few specks onto a damp white paper towel. Flea dirt is flea droppings made largely of digested blood, so it smears reddish-brown when wet, while ordinary dirt stays black. Finding it means fleas are present even if you never see one, since cats groom away the adults. Check the base of the tail, the back and the neck, where it tends to collect. If the smear test is positive, treat with a vet-recommended flea product and don’t forget the home, since most of the flea life cycle lives in the environment.