Suppose that we define zombies as a fully functional humanoid creatures who lacks subjective phenomenological states, including inner thoughts and feelings. These are a tad different than the zombies you may be familiar with from popular culture. We could say of such a creature that it lacks an “interior life,” or introspective states. Despite that zombies have no introspective beliefs or feelings, they are still able to respond to external stimuli as anyone would expect of someone who had internal goings-on. And they behave in ways that are consistent with having a robust interior life. This means that they can fool the average person into believing that they have a robust interior life, i.e., that they have thoughts and feelings.
We want to be able to distinguish the living from zombies because the living are truly alive while zombies are only partially alive. There is no doubt that the living exhibit behaviour consistent with what we would expect from someone who is living. For example, in walking down the hallway, I gesture toward Bob and say, “hello Bob!” Bob acknowledges my gesture and my greeting with his own, e.g., he waves and says, “hello Joe!” The living are able to “acknowledge” and they are able to “greet.” They can reason about problems, experiential or otherwise. But, because zombies behave in ways that are consistent with the living, we should expect the same from them. What would be missing from the zombies' acknowledgement and greeting would be the inner state, whatever that may be. Zombies will exhibit behaviors consistent with “acknowledging” and with “greeting,” but there will be no accompanying inner feeling. They too can display behaviour that exemplifies reasoning about problems, experiential or otherwise.
Except, the problem is that the living are actually reasoning and living, and zombies are merely behaving in ways that make outside observers believe that they have a robust interior life. If they engage in these sorts of behavior, then we cannot distinguish zombies from non-zombies. The behaviors of a zombie are consistent with the behaviors of a human. So, there does not seem to be a way of distinguishing zombies from non-zombies.
Perhaps one might argue that the definition of zombie begs the question about zombie individuation. By this we mean that the definition itself prevents us from distinguishing zombies from non-zombies. If zombies are creatures without an interior life and an interior life cannot be identified by anyone but the person who has them, then zombies would have to identify that they have an interior life. But zombies by definition don't have introspective states. If zombies were to identify having internal mental states, then they would be merely acting like non-zombies. If the criterion of individuation is difficult, if not impossible, to falsify, then we have to question whether the criterion itself is mistaken.
This could be true, but I gather that individuating zombies from non-zombies will have to point out something about “being internally dead, though they appear to be alive.” How do we determine whether something’s going on inside the head? Being internally dead would require the individual to say that they are internally dead since there would be no means or tool we could use to identify that something is going on inside the zombies' head. For the internally dead zombie to identify themselves as having something going on inside their head would seem to work against having made such a claim. If the person were able to identify that they had no internal states, then they would be at the same time having internal mental states. Their mental state would be one that identifies that they do not have such internal mental states.
One might think that x-rays or fMRI scans would help us individuate zombies from non-zombies. These scans would be evidence of internal goings-on. Even taking pictures of brain function wouldn't help here. Undergoing an fMRI and identifying where there is brain function would be an outward manifestation of brain behaviour that is consistent with the definition we provided of a zombie above. Of course, that external behaviour is consistent with the zombie not having any internal mental states. The behaviour is present but the internal feel or phenomenology is missing.
One might try to individuate zombies from non-zombies by using the appearance of being alive. The appearance of being alive is not to actually be alive. There’s something about actually living creatures that distinguishes them from zombies. The criterion might be that they have fully functional internal organs. Fully functional internal organs suggest that they have some sort of internal goings-on. Thus, zombies and non-zombies are distinct. Yet, even these distinguishing features would be something that can be accounted for by the zombie advocate. The appearance of being alive is part of the deceitful behaviour of zombies.
Since zombies exhibit the same sorts of behaviors as humans and a feature of humans is that they have fully functional internal organs, it seems that this criterion fails too. The zombie’s innards should be revealed to be the same as the humans. That's just what we should expect.
Is there a means of distinguishing the living from the living dead? If there is a way of individuating them, it doesn't seem to be apparent how we would distinguish between them if our only means of analysis is behavioural output. We are left to wonder what criterion will suffice to individuate zombies from non-zombies. And no amount of investigative work or accumulated evidence will provide us with alternative theses concerning the internal goings-on of an individual. We'd merely be left to guess whether what the person's reporting is an accurate read back of their mental states.
I suppose that one may ask just what hangs on the existence of zombies. Even if there are zombies that live among us, we would not care whether there were any. They would still function in the same way that we do, and they would interact with us in normal ways. So, there wouldn't be a reason for thinking that having zombies around would have catastrophic consequences for mankind. That they exist wouldn't take away from my quality of life (or my life, for that matter!), and they wouldn't take away from their own quality of life. They would merely be a shell of a person without an interior life.