What are the signs of a stray cat?

Well first check for a collar, microchip or a tattoo (you need to take it to the vet to check for the microchip) if it has one it probably belongs to someone. Just put up some found posters and check the shelter and local vet to see if there are any missing cats resembling him. If no one answers he was probably a stray or released by his owner. You will be free to keep him!