In practice, I'm not sure there's anything that can be done about service dog misrepresentation.

When I was trained in restaurants to deal with (potential) service dogs, the extent to which I was allowed to ask any questions was "Is that a service dog?". As long as the answer was "Yes", I was to leave them alone. No questions asked. Anything else supposedly opened you to litigation

Edit: Looks like there's a separate thread with additional ADA info. Tl;dr - nothing you can do to stop people misrepresenting their dogs as service animals.