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.''

Can we conclude that you can tell a lot about a person by how they treat their waiter?
yes