Zombies Diagnostically

 The key feature of fake-alive mortals is they cannot see anything new. They can only dream of things they experienced when they were alive. 

 They suffer Plato's anamnesis, meaning anything they can't un-forget is something that's invisible to them.

 If what they need is something they have not previously eaten, then they will starve. If the solution is not to a problem they have already solved, then they have no choice but to tolerate the problem. 

 Sometimes it appears as if they're fooled by appearances, but if you radically change the appearance, they will continue to see the same phantasm in their mind. To them the substance is ontologically inaccessible. At best, they can swap the old coat of paint on an old idea with a different old coat of paint, to make something superficially novel.

 They are wholly cut off from external reality. You may recognize the resonance with narcissism. They can't see anything that isn't an image projected from inside themselves. They are blind. They are deaf. They cannot smell or touch or taste. Anything which doesn't already dwell inside them functionally doesn't exist to them, except that it can kill them. 

 When I meet a zombie I instantly intuit the things they cannot see. What sights do their cataracts block? I already know. Necromantic skills.
 If you can't perform necromancy and need to test more manually, I'm not sure how you would feasibly do it. Perhaps serendipity: be aware that if someone seems resistant to an idea, it might be because they're already dead and cannot read it. If they cannot acknowledge that the idea is new, if their mind keeps snapping back to the nearest pre-existing thought, then you've found a zombie. Stop trying, it is impossible for them to hear speech. 

 Another necromantic skill: catalogue the zombie's finite list of brain states, and induce it to pick the one most personally convenient to you. It cannot think the truth, and consequently it can't not be deceived. Cooperation is  impossible, so choose defection. That said, zombies stink, even the ideal brain-state is worse than the zombie not being present at all. Don't build a herd of zombies, induce the zombies to leave you alone.

 Conveniently, all black government agents are zombies. Seems to be karmically guaranteed. They always have blind spots. Rather, whole blind fields, across which you can safely cavort. 

 Corollary: being unable to think anything new, they can't change their behaviour without a vampire or necromancer.  "And stuff that he doesn’t like about me is also never gonna change, like leaving coffee cups all over the house."

 How hard is it to pick up a coffee cup when you're done with it? Maybe take it with you? Even a dog could learn to do this. For zombies, however, the [learning] part is insurmountable. A necromancer can force a rearrangement of the dreams - you've picked up other stuff, pick up this stuff too - but the necromancer will regularly have to use a whip to accomplish it. 

 

 These features add up to making the zombie seem noninteractive, unless you count shutting them down, lol. You can opening up the back panel and fiddle with the wiring, and just like a nonalive game NPC, they will start up again with no self-awareness of the change. Unless you deliberately wire that in, I suppose.

 Conversely, non-zombies take a penalty on IQ tests. The correct answer to a raven's matrix is, "This is stupid and nobody cares." Noticing this answer takes time, and IQ tests are timed. Forgetting the real answer and trying to work out what the question is trying to ask takes time. Only zombies can mindlessly follow orders, meaning, properly speaking, the tests have to be normed differently depending on whether you're testing a zombie or non-zombie. IQ tests are created by Nameless worshippers, meaning they assume everyone is identical. Except for IQ, because Nameless worshippers can't into consistency. Most mortals are zombies and consequently IQ test-makers assume everyone is a zombie.