That's not true; not all service animals wear vests, nor are they required to.
If it is not obvious that the animal is a service animal (they can also be miniature horses, not just dogs!), you are only legally allowed to ask two questions:
1) is the dog a service animal required because of a disability?
2) what work or task has the dog been trained to perform?
This publication provides guidance on the term “service animal” and the service animal provisions in the Department’s regulations.
www.ada.gov
That's it. Can't ask for papers. Can't ask for a demonstration of the animal at work. And that's probably not even questions Target wants you to ask anyway.
Little known fact, even if a legit service animal, you can legally ask a Guest to remove the animal if the animal is not exhibiting control or if it's not housebroken. You just have to help them shop
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From the same website above...