Flea dirt looks like tiny black specks, similar to ground pepper, and it’s often found on pets’ skin or in areas where they rest. To test it, place some of the specks on a damp paper towel — if they turn reddish-brown, it’s flea dirt (digested blood), and you likely have fleas nearby.
https://specterservice.com/how-to-check-for-fleas-without-seeing-them/

