Leave a full description of the pet, and the area in which you found it. Check the newspaper classifieds for lost pet ads. Place a found ad in the newspaper, even if animal control has already taken the pet. Some papers will even run free found ads. If you are interested in keeping the animal, please check your local laws first.
Some states consider keeping a stray without following the appropriate ordinances as theft.
Your animal control officer or animal shelter will know the legal procedures to follow to make that pet yours.