The Waiter Rule refers to a common belief that one's true character can be gleaned from how one treats staff or service workers, such as a "waiter". The rule was one of William H Swanson's 33 Unwritten Rules of Management, copied from Dave Barry's version "If someone is nice to you but rude to the waiter, they are not a nice person."
References
Waiter Rule Wikipedia(Text) CC BY-SA