Another thread prompted this thought. Sometimes, through no fault of your own, your critters can escape and go missing. Here are the ways I that I identify my own dogs so they can be returned should they decide to "go walkabout."
Collar tags are ok, but shouldn't be relied upon as the only source of ID on your pet. They can easily be pulled off as your critter escapes under the fence, runs through brush, &c. It is such an issue that in Alaska, state and local AC ordinances make an exception to the requirement for rabies tags for working dogs so long as the owner can produce the original certificate of vaccination. I've never been able to keep a "dangly" tag on any of my dog's collars for more than three months.
A better solution is to rivet an identification plate to the animal's collar. It's a more permanent solution, but even that requires some attention and maintenance. I've haven't yet had a dog loose such an ID plate, but I have had to refasten them from time to time when one end or the other has come loose.
I buy the ID plates I use a dozen at a time (I keep 10 dogs, currently and plan to have more within a few months). The only information I put on the ID plates is my kennel name and my phone number.
As a second line of defense, I print my phone number directly onto the collars with a permanent ink marker. That way, even if the ID is somehow lost, immediate contact information is provided. Every few months it needs to be refreshed, but it's quick and easy to do so.
Finally, all of my dogs are microchipped. Today most veterinarians and animal control departments have universal chip readers that can read any of the more common ID chips.
If you acquire a dog that has been "chipped" by someone else, be SURE to ask for and fill out the paperwork changing the owner of record with the chip registry. It's important that you be the owner of record. Here's an example of why that is important.
Some time back I captured a stray geriatric sled dog that had wondered into my yard. I hooked him to a spare tether and started looking for ID. There was nothing on his collar and no ear tattoo that I could find, so I fed him along with my guys and the next morning took him to AC to have them scan him for a chip.
He had a chip, issued by one of the long-distance sled dog races up here. A phone call to the race headquarters gave us the name of the dog's owner. When we called the original owner, we were told the dog had been sold to a different musher. Fortunately, he remembered who he'd sold the dog to. We called that person, who told us he had given the dog to another musher who was trying to start a sled dog team.
That musher had run the dog for two or three years, and then retired the dog to a pet home, but couldn't remember the name of the person who adopted him, and had no contact information for the person.
So, having spent most of a day playing phone tag, I still ended up relinquishing the dog to a local rescue group, who placed him in a different retirement home. It was very frustrating.
On the flip side, earlier this spring one of my neighbors caught an errant Siberian husky who showed up in her yard right at feeding time. She called me and asked if I'd help her find where he belonged. I took him to town, had him scanned, and we got a "hit". They were rather surprised that the registry had already changed their records, as they acquired the dog already chipped less than three weeks before.