Genuine service dog or not?

This morning I had the following message from a friend who runs a major musical venue:

"I have done much research and followed your postings about service dogs and feel that I have a good understanding of the laws and the need and importance that the dogs have in people's lives.  On the other hand, we have many people wanting to bring their dogs [into the venue], and in the back of my mind I am often skeptical when I see people with dogs, wondering if they are just sneaking them in.  

"There was a man at the venue with a large dog in a harness, and he was walking with him all over the grounds.  At times the man was drinking.  I had convinced myself he had probably snuck the dog in, but accepted that there was nothing I could do about it except to obey the law. It nagged at me every time he walked by.  On Friday I saw the dog lie down by a child, and I could see the man talking to the child and a parent and I thought they were petting the dog.   

" 'Service dog, my ass,' I was thinking again. Then, on Saturday evening when our headliner was on,  I saw the man walk by with the dog and a beer in hand. I leaned over to the security volunteer next to me and said to him sarcastically what I had been thinking all weekend: 'Yeah, that's a service dog. Sure it is.'  

"This I will never forget. The security volunteer looks at me and says, 'Diabetes. The dog is trained to detect low blood sugar. Did you hear what happened?'

"To which I said, no I had not. He went on: 'The guy was walking by a child and it performed its service dog task. [Presumably the dog smelled a chemical change in the child's breath and nudged with his nose.] The man asked the parents, "Does your child have diabetes?" To which they replied, "Yes.'" He told the parents, "Give him something. His sugar is low." Sure enough, it was.'

"Shame on me for my thoughts and snarky comment. This was a real eye-opener and one I will share with my staff when we have our next meeting. 

"That is exactly the reason we are only able to ask the permitted questions ["Is that a service dog?" and "What task does he do for you?"] and not make our own judgments.  I was ashamed of my negative thinking and will never do it again. Big lesson learned.”

Yup. It's nearly impossible to tell a genuine service dog from a fake, especially on first sight. Some disabilities are invisible. Best thing to do is just go along and assume the dog is kosher.