How dow we know that zombies don't exist? How do we tell the difference between zombies and humans?
Almost no-one with a decent education seriously believes that there are zombies roaming the streets, unless perhaps you happen to come across the filming of a science fiction movie or it's Halloween.
If I were to encounter someone who tried to argue for the existence of zombies I would treat their arguments in the same way as I treat the arguments for the existence of god(s). They are impossible to completely disprove, but there is no good reason to believe that they are true. Furthermore, if someone did believe in zombies (or BIV as a true picture of reality) then I would assess their reasoning skills as deficient. Hume's rejection of miracles tells us that they are probably under a misapprehension.
Such beliefs are a good touch-stone for assessing whether someone has mastered the scientific method. It also tells us whether they have "grown up" in rational terms and moved on from pre-historic theist or deist myths.
I concede however that I am not really addressing the question of what criteria I use to judge that zombies don't exist.
I suspect that the reason I do not believe in zombies is inherent in what we mean by "zombie". A "zombie" is not a real entity, at least as far as I am concerned. The origin of zombie as a word seems to go back to West Indian origins and is now often associated with Haitian voodoo beliefs. As with all religious stories, they were invented to scare the credulous into submitting to the power of priests.
So the criteria I use to judge if zombies exist is the same criteria I use for all beliefs in the supernatural. In a sense I don't need any criteria, the default position is that they don't exist and I am unlikely to give any serious consideration to the possibility that they do exist.