This be a stickly one. After three years of studying it , I cannot tell you a good answer. Especially since I am genetically predisposed to failure of Tact 101. Directly pointing at the animal in question and laughing just doesn't work. This approach has worked for me, and, has ALSO NOT worked for me.
Humans are funny people.
Some Folks would rather believe the lie, and keep it alive, than accept the truth, even if the Truth bites them right on the butt. It also makes a difference WHY the person has the animal. So much ego seems wrapped up in wolf content, and coupled with the fact that many people, owners included, have little experience with real wolfdogs or wolves, makes convincing these owners their animals are misrepresented a most daunting task. Probably the best tactic I have found is to put together a photo collection of physical traits of wolves, wolfdogs of known and verifiable content, and various breeds or mixbreeds of dogs. I have used such a collection to educate owners.