SERVICE DOGS are dogs that may or may not be wearing a service dog vest. Please don’t interact, make eye contact with, or pet this dog while it is working. When in doubt, ask.
A service dog is a type of assistance dog specifically trained to help people who have disabilities including visual difficulties, hearing impairments, mental illness, seizures, diabetes, autism, and more. (source) Service dogs are highly trained to perform a specific function. Training can cost anywhere from $7,000 to $20,000. Properly trained service dogs are not reactionary animals and won’t be distracted from their task by strangers. Many dogs don’t make the grade because being a service dog is such an intense job; the standards are rigorous and very demanding. (source)
Service dogs are free to act normally when they are not working. Typically, the animals are taught to identify work versus free time by whether or not they are wearing their gear—if, in fact, they wear gear. Exceptions to this rule may exist, such as a seizure alert dog, which must not ignore an impending seizure even when it is not wearing its gear. Nevertheless, just as with any other trained animal, working dogs must still obey commands even when they are off-duty. Because of the strict behavior expected from a working dog when it is on duty, many owners will usually not permit people to pet the animal, or are reluctant to remove gear on request, such as for security inspections. (source)
Disabled owners of service dogs are protected under the Americans with Disabilities Act of 1990, which generally gives them the right to be accompanied by their service animal anywhere the general public is allowed. Additional federal laws protect people with disabilities partnered with service animals, as well as other types of assistance animals, from discrimination in housing the Fair Housing Amendments Act and on aircraft (the Air Carrier Access Act). (source)
The simplified version with list of major points to remember (read page). What businesses need to know about service dogs (read page). When is a service dog, not a service dog? (read page). Spotting fake credentials (read page).
Various organizations train dogs for service or assistance work. Call or email them and verify they are still available.
Additional reading, the ‘Service Dogs for Veterans / some for civilians as well‘ Facebook page published by a veteran.