I don't have a disability, other than a variety of health concerns, that no one can see and for which I do not use a dog. So I don't get this question.
People will ask things that are none of their business, and while you can ignore them or tell them flat out that it is none of your business, I don't think it is helpful to service dogs and their owners in general to promote animosity.
You can use it as a teaching moment, by saying something like, "I'm sorry, the service the dog performs is personal, like health is personal."
I mean, one of the reasons dogs are great for emotional support, is because people can accept complements or converse with perfect strangers when it is about the dog. There is a freedom associated with dogs. You would pet someone's dog much quicker than giving a friendly pat to their kid's head.
Dogs are amazing, and seeing a service dog in person when we have seen them on specials on tv and such, well, they are an attractive nuisance. I saw a special on dogs that sniff out cancer, and I've seen other dogs used for cadavers. Yeah, I would probably ask if I saw a dog in the exam room, is that a medical dog? What does he do? Does he sniff out cancer?
For stangers who have a service dog vest on their dogs, or have a dog in a grocery store, I think most of us have been taught to not pet, and try not to notice, so we don't make people uncomfortable. The thing is, not seeing disabled people is just another way of noticing them and making them uncomfortable. So I don't know.
Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.