This isn't true, by the way. Businesses aren't allowed to verify the legitimacy of a service animal because that's seen as discrimination, which is most of the problem in itself as it allows any random person to buy a $10 vest from Amazon and use it as a "bring animal everywhere free" card.
But there's nothing prohibiting random people like you or me from questioning the legitimacy of a service animal, and if someone gets super defensive about that question, odds are good that it is indeed a fake service animal.