Instructions
Look at the dog for any sort of accessory that states it's a service dog. Certain certification programs have their own badges, accessories or logos. Sometimes the owner of a service dog might make or purchase an accessory.
Watch the dog's behavior. Service dogs are not like pet dogs; they are highly trained to do various tasks and are well behaved. A service dog should not try to greet new people the way pet dogs do. Instead, the service dog should remain at the owner's side at all times. Some tasks a service dog might have are being a Seeing Eye dog, picking up items that drop, offsetting the owner's weight to help with walking, opening doors or fetching frequently needed items. Not all service dogs have tasks like this. For people with psychiatric or mental disabilities, the dog may simply be there to comfort.
Ask the person (if you are a business owner) in a polite and calm manner if the dog is a service dog. Do not under any circumstances be hostile or defensive about the animal, even if the person has no identification or certification for the service dog.
How to Recognize a Service Dog
Service dogs accompany people with various types of disabilities, some physical and some mental. While many service dogs may wear a special badge, bandanna, harness or other accessory, it is not required by law to have identification or for the dog to be certified. It is also illegal to inquire as to the person's disability, although you may ask if the dog is a service dog. If you are a business owner, you are also required by law to let any service animal into your establishment, even if the local or state health code mandates no animals. The Americans With Disabilities Act trumps state and local laws and codes.