While no special certifications for service dogs are required by the Americans With Disabilities Act, you may wish to get certification to show the dog's training to assist with specific disabilities. You'll find reputable organizations in the United States that certify dogs as service animals.
While the Internet is filled with registries offering ID tags and documents "certifying" a service animal, getting an animal truly certified involves much more than sending money and your dog's photo. Look to national organizations such as Assistance Dogs International, which has member chapters throughout the U.S. The organization uses a standardized ADI Public Access Test showing specific commands, tasks and level of control to certify graduating dogs and their special needs person. Some allow owners who have trained their own dogs to become certified after passing the test. The American Kennel Club also maintains a list of organizations offering certification for service and therapy animals. Certification among the organizations is not standardized, with each having its own set of requirements. Check the state where you live as they also may provide certification and the accompanying paperwork.