In North Carolina if you find a dog and take care of it for 2 weeks it's considered yours. I had to call the animal control office when I found two dogs on my back porch on Christmas day. They were strays and I was trying to find the owner or a new home for them. The animal control officer asked me if I was feeding them and I said "yes" he asked how long and I said "two weeks" he said their yours. You can't keep a stray longer than 2 weeks or your responsible for them. I didn't know that. They wouldn't come and pick them up so I had to take them to the animal shelter. (since they were considered mine) That's what you get for being a good samaritan. You can also go to www.lostfoundpets.us and post it there
A good way to tell how old a puppy is, is the eruption of adult teeth. adult incisors usually erupt at about 12 weeks of age, and they lose their baby canines at about six months. Your best bet would be to take the pup to the vet for vaccinations (parvo is an extremely deadly disease that requires a series of vaccines to help boost the immune system.) and let them take a whack at it. As far as your rights, i would talk to your local animal control officer.
I think you should place your own ad- most newspapers will run a "found" ad at no cost. my friend located her runaway last week this way. you should give the owners every opportunity to find their dog.