Many people are reluctant to take a found pet to the humane society or local animal shelter when found for fear the pet won't be reunited with its owner and be euthanized? Start by checking the pet for a tag or microchip. Tags should be obvious as they would be on a collar. Most humane societies or emergency clinics will have a scanner and will scan the pet for you to identify a microchip. If they find one, they will try to contact the owner for you.
What else can you do? If you do take the pet to the humane society or animal shelter, you can leave your contact information with a note that if the owner is NOT found, to call you as you will consider adoption. From there, you can decide what you want to do.
Alternatively, here are some other things you can do to try to find the pet's owner.
Contact your local humane societies, animal shelter, local veterinarian's office, and local animal emergency clinic. Fax or take them a flyer to post on their bulletin board.
Create a flyer. What is in the flyer? Information about where the cat was found, the date it was found, a description of the pet, a photo of the cat and your contact information should be included.
Place an ad in your local paper or papers (since it is free!)
Posting signs or a flyer in the neighborhood where the cat was found (posting photos with the information I mentioned on telephone poles, your local library, grocery store, ice cream parlor, etc).
Walk to neighborhood. Take a walk and ask around if anyone lost a pet.
An article that might be helpful to you is What to do if your cat is lost. Although it is written for those that lost their cats, it has helpful suggestions on where they might be looking to help guide your efforts.