The last time I looked up the statutes, it said that the only legal questions you could ask someone with a service dog were, 'Is that a service dog?' and 'What service is it trained to provide?' And those questions are only legal coming from a business owner. This may have changed since then.

ADA says you MAY ask but if the dog is behaving, quietly, not causing a scene it's a trained SD. My SD has been attacked by ESD by people saying it's a service dog.