Here is the link to the FEDERAL standards for Service Animals:
beta.ada.gov/topics/service-animals/
You will want to read your specific state standards as well. But according to the Federal standards, the disability is associated with the person. Ask the person to show their documentation from a licensed physician verifying their disability and they should also be able to explain and demonstrate what specific function the dog can perform for them, associated directly with their disability. I haven't found any government licensing for service animals on the state level. Your state may have a broader definition of service animal, to find that out, go to your individual state's attorney general office website for more info.
Here is an exerpt copied directly from the federal guideline on Service Animals:
"Service animals are defined as dogs that are individually trained to do work or perform tasks for people with disabilities. Examples of such work or tasks include guiding people who are blind, alerting people who are deaf, pulling a wheelchair, alerting and protecting a person who is having a seizure, reminding a person with mental illness to take prescribed medications, calming a person with Post Traumatic Stress Disorder (PTSD) during an anxiety attack, or performing other duties. Service animals are working animals, not pets. The work or task a dog has been trained to provide must be directly related to the person’s disability. Dogs whose sole function is to provide comfort or emotional support do not qualify as service animals under the ADA."