If you have found a loose/stray dog or cat, this article is a must read!
Missing Pet Partnership’s “I Found a Stray Dog.” Much of this is also applicable to found cats or other pets.
Check for a Microchip
Find someone at a veterinary office or shelter who can help you check the dog or cat for a microchip. Most often the vet or shelter will be able to look up the microchip information if one is found, but you can also use the AAHA Universal Pet Microchip Lookup site to determine which company the microchip is enrolled. This can also be done with just the microchip number if the dog or cat is wearing a microchip ID tag. Then you just need to contact that company and give them the microchip number and they will either give you the pet owners number (if they have permission to do so) and/or contact the pet owner themselves.
Check the dog or cat for any ID tags or a phone number embroidered into the collar. Rabies tags and licenses can also be used to track down the owner. For a rabies tag, call the vet listed and give them the number to look up the owner. For a dog license, call the Animal Control Officer or Town Clerk of the town listed on the tag and give them the number. In either case, they will either give you the pet owners phone number or more likely ask for your contact information and attempt to contact the owner themselves.