Many people are reluctant to take a found dog to the humane society or local animal shelter when found for fear the dog won't be reunited with its owner and be euthanized. Start by checking the dog 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 dog to identify a microchip. If they find one, they will try to contact the owner.
What else can you do?
If you do take the dog 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 dog'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. Include information about where the dog was found, the date it was found, a description of the dog, a photo of the dog and your contact information.
Posting signs or flyers in the neighborhood where the dog was found (posting photos with the information on telephone poles, your local library community board, at the grocery store, ice cream parlor, etc).
Place an ad in your local paper or papers (since it is free!) Even if the owner doesn't read it - a friend of theirs might!
Walk to neighborhood. Take a walk and ask around if anyone lost a dog.
An article that might be helpful to you is "What to do if your dog is lost". Although it is written for those that lost their dogs, it has helpful suggestions on where they might be looking to help guide your efforts. What To Do If Your Dog Is Lost.