There is absolutely NO way you can tell at a glance that a dog is a service dog. Please oh please stop assuming it.
I get that people have the image of a Seeing Eye Dog in their minds, with the harness and the vest and all of that, but that is ONE kind of service dog.
Today I met three massive Newfoundlands who were service dogs. Their owners shared with me that they are seizure dogs; they can detect an oncoming seizure. The woman shared that her son is a veteran and had come back from Iraq with traumatic brain injury and seizures. These dogs could tell, without training, when his body was setting up for a seizure. They alert him which lets him get safe. She said that they will surround him and keep his head from hitting the floor. The dad said that the dogs will come in and wake him and his wife up, if their son is having problems while he is sleeping.
Amazing, isn't it?
And many small dogs, the ones that are carried, are just that sort of service dog. Some dogs can also sense blood sugar changes before the person actually feels the changes or regularly tests. (I don't know how it works with type 1; DH had type 2 for a few years, and it was super-obvious when he was going high, because he changed so much, and when he was being honest with himself he could sense it, too...I don't know if a dog could sense it BEFORE we would notice it, would have been interesting to find out, but he made the diet and exercise changes needed and doesn't have it anymore) The Newfie owner said that they later realized that their dogs can sense the blood sugar problems, too. And apparently they will alert for *anyone* around them who is having an issue. (eek!)
So you simply have NO way of knowing that a dog is not a service dog. I choose to assume that a dog IS. And I choose to be happy for the person who has the dog with them, because they are getting the help they need. How wonderful is that? This combo makes my life happier. I recommend it.