Methods of identification in dogs:
https://www.petplace.com/article/dogs/pet-health/methods-of-identification-in-dogs/
Some form of identification for your pet is vital. Of the millions of dogs and cats euthanized in shelters around the country, an estimated 30 percent of them are lost pets whose owners cannot be found. Shelters only hold “stray” animals for a short time – sometimes only for a few days. Without identification, they are inevitably euthanized unless adopted out. Identification has evolved over the years, from collar tags to tattoos and, more recently, implanted microchips. All are still available, at a reasonable cost. But which one is best? The short answer: combine the traditional collar tag with either a tattoo or a microchip. The reason is that the average person who finds a lost dog may not know to look for a tattoo and won’t be able to detect the microchip without a scanner. Often, the effort to contact the owner depends on how easy it is to do so. Collar tags can provide immediate contact information. Along with the ID tag, your dog should wear his license, which indicates that he has been vaccinated against rabies.

Here are the individual advantages and disadvantages of different forms of identification:
Collar Tags:
These are a must. Tags immediately tell a shelter worker that this dog has a home and a family who miss him. Tags should have up-to-date information on the name, address and phone number of the owner, and the name and number of the veterinarian (if there is enough room).
The ID tag should be accompanied by the dog’s license tag, which tells whoever finds the dog that he has been vaccinated for rabies and is registered with the local government.  The tags should be attached to your dog’s collar. Unfortunately, collars can be lost or (if the pet is stolen) taken off, so you’ll want to have a backup method of identification. If you don’t like ID tags, you can get collars that come with a plate to engrave your information.

Top Tag Pet ID:
The Top Tag Pet ID consists of a small 64MB flash drive that comes in a waterproof case and attaches to collars. The program on the flash drive is filled with easy-to-access information and photos. The program allows you to write, organize, store and retrieve all of your pet’s important care information. This tag system allows you pet to be identified if he is lost and also allow pet sitters and rescuers to know how to provide the best care for your pet.

Pet Protector System:
The Pet Protector System is a 24-hour, nationwide emergency system much like Medic Alert is for humans. All vital information about your pet is instantly available to the company’s emergency operators. Should your pet get lost or need emergency assistance of any kind, whoever finds your pet simply calls a toll-free number located on a special tag on your pet’s collar and a trained operator is there to coordinate the assistance your pet needs. For more information, go to http://www.help4pets.com.

Tattoos:
Tattoos have been used to identify dogs (and cats) for many years. For some, a series of numbers and letters are tattooed on some part of the animal, such as the upper rear leg.

That number is registered with a company such as National Dog Registry or I.D. Pet, which can contact the owner. Some purebred dogs registered with the American Kennel Club are tattooed and registered with the organization.
The owner can also have his or her contact number tattooed on the pet, but this can obviously cause problems if the owner moves away or changes numbers. In addition, that’s a lot of data to tattoo on your dog or cat. The efficiency of tattoos is under debate. Some people say they are reliable, others say there have been cases where the tattoo had become illegible.
There is also the problem of location and convenience. The average person would have to know to look for a tattoo, then find the number to a registry to contact the owner. (The best course for that person may well be to contact the local shelter and inform them that they have found a tattooed dog.)

Information specific to letting people know your dog has ME:
Some ME dog owners prefer to have something saying they have Megaesophagus so are special needs. They include emergency contact of themselves but also their veterinarian. Others have noted that since most people who might find a lost dog won’t know what Megaesophagus entails, so not worth taking up time with that specific info. Only problem is IF the dog needs to eat, be hydrated, or see a veterinarian, they NEED to know it is not necessarily safe to perform normal test and evaluations on one of our dogs without following some precautions. What they do may cause more harm and risk to the situation. Err on what you feel is safest for your dog and your particular location. One owner uses the following info on tags for her ME dog: He is wearing his microchip ID tag, tag with name and phone numbers printed on both sides, one tag that says “DO NOT FEED” and “SPECIAL NEEDS” on the other side and another tag that reads “MEDS REQUIRED” and “DO NOT FEED” on the other side..

Dogs should:
–   Wear a collar at all times
–   Be microchipped with some sort of tag indicating this attached to collar
–   Have identifying tags of some kind attached to collar
–   Have proof of rabies vaccination or something that provides your veterinarians contact info attached to collar

IMPORTANT: Having your veterinarian also mentioned allows whoever would find your dog to get health status more quickly plus background of special needs. As for the rabies vaccination, many of us opt not to have them vaccinated pending how badly doing so affects them. We use a titer which would be shown on their health records too.