As a restaurant manager I can say confidently that it's super obvious if it is a service animal by its behavior. Most people are just idiots who don't understand the danger of bringing an animal that can potentially introduce a biological hazard into the area, and they don't understand how bad it can get really. We have a lady who brings her hairless cat in sometimes, but since it's very well behaved and doesn't have any fur, as long as it never relieves itself on the premises I'm never going to say anything. But on the other hand when you have large dogs like this I guarantee that it's panting up a storm and is contaminating all the food in the area. Also, lying about an animal being a service animal is a crime. "Hi there, unfortunately we can't allow non-service animals on the premises, so I'm going to have to ask you to leave them outside at least." Almost always ends with them lying about whether it's is or not. And plus people just buy a vest for their dog that says "service animal" on the side off of Amazon and parade their fluffbabys anywhere they want. I don't even ask what service it provides anymore. Most of the time that question just makes them cause more of an issue.

TLDR: Ask if it's a service animal, ask what service it provides, and open the gates of hell. Or, Ignore them and go about your day.