If the dog is at all unusual, it is a good idea to show the dog to a person who works with that breed in rescue. For example, recently I was shown a large white dog that some quite responsible people had adopted from a shelter as a Great Pyrenees. They had continued to try an locate the owners. However I think the dog may be a Kuvasz which would mean that much of the effort in finding the owners would have been wasted. Before you jump to the conclusion that the dog was severely mistreated make sure you know the breed.
Many people think perfectly normal greyhounds, whippets, salukis and borzois are starved.
They decide not to look for the owner because of the perception of abuse. These dogs are naturally thin. Modern Irish Setters and German Shepherds can also be thin. Remember to post notices of your find in the area where you found the dog and to phone shelters in that area and to run ads in a newspaper local to where the dog was found.