My main character lives in a world of magic. Nothing seems off about the people around them, but some of them are Philosophical Zombies, automatons without any inner life, for reasons important to the plot. What non-contrived ways could the character discover this fact without any doubt? Feel free to conjure any magic you want. It's more of a sequence of events that lead to the discovery I need.
The whole philosophical point of zombies is that there is no way to distinguish them from outside.
Which obviously means that you need to distinguish them from inside. You need some kind of magic that lets you get inside the minds of other people—whether it's telepathy, or possession, or whatever.
At which point it becomes trivial: you get inside their mind, and there is no "inside" there.
It's hard to imagine how to stretch that out into "a sequence of events", short of by developing that magic over time. For example:
- At first, you can just borrow the sense perceptions of other people (and animals?), and the zombies are the same as everyone else.
- Then you learn to feel the effects of… not emotions, but the hormonal and other physiological changes that go with them, and still the zombies are the same.
- Then you learn to read their memories, and even use their minds to figure things out for you, and still the zombies are the same.
- Then you learn to access their thoughts, and… some people are just different in some way you can't explain.
- Then you learn to access their second-order thoughts, their self-reflective stream of consciousness, and now you get it: the zombies don't have one.
If you can also control people, the way you control regular people and zombies will be very different.
With regular people, you essentially insert yourself into the arguments happening inside their conscious mind all the time and sway the "vote" the way you want, and the decisions and first-order thoughts and actions all follow from there. But this is more complicated than it sounds, because people are making decisions without conscious deciding all the time, and you have to sway them indirectly.
But with zombies, you just directly produce an order and it's followed. At first, you'd think this makes things easier—but in fact, because the orders have to be at a much lower level than the kind of things we determine consciously, it's actually a lot more effort to control a zombie in any useful way.
Maybe this leads you to realize that whatever's using the zombies must be either a pack of people or a superhuman force (a computer-god or something?), because otherwise they'd be useless.
If you've read Julian Jaynes and find his Bicameral Mind hypothesis interesting, you can put a twist on this—which doesn't really fit the use of zombies in philosophy, but might be more interesting for your story anyway.
Conscious people, whose left and right hemispheres are integrated, are just like the above, and you have to get involved in the discussions inside their brain to do anything useful.
Zombies are people whose hemispheres are independent. Their right brain issues verbal commands; their left brain follows them, using its intelligence to do so as well as possible, but unable to even think of disobeying. Zombies believe these commands come from the gods. This makes them different from non-zombies, and philosophical zombies, because they actually describe their motivation completely differently. Why did you build the house there? Marduk told me to build the house there and so I did.
Anyway, this means you can read and control the left brain of a Jaynes-zombie quite easily, putting yourself in the place of their internalized gods. But it also means that if you want to understand, or control, their deeper motivations, you have to talk to the right brain, which is like a cacophony of gods fighting over who gets to give the next command to the left brain. These gods are, of course, not superhuman, but in fact less than human, but that doesn't make them much less interesting than squabbling Olympians. And you've got a whole pantheon of them for each zombie—similar to, but independent of, each other.
Challenge the P-Zombies to any kind of activity that involves self-reflection or (mental) self-improvement. Free will means more than just that we can decide to do what we want; it also means we can decide not to do what we want (I can decide not to have that piece of cake, despite the fact that I want it) and moreover, I can (with effort) decide not to want what I want. We can in fact train our mental virtues -- self control, patience, courage, etc. -- the same way we train muscles, heart, and lungs.
I would suggest that philosophical zombies would be incapable of self-reflection. They could tell you why they're doing what they're doing -- "I did X because I want X. I want X because I'm a Y kind of person and I value Z" because they're sort of programmed with this motivation, and may be programmed to give that explanation for it. But the zombies wouldn't be able to change their motivations. If you asked them what they like or dislike about their motivations (or personalities) I think they'd be dumbfounded by the question, incapable of reflecting on an interior life that doesn't really exist.
Your main character might discover/realize this while attending a meeting of a self-help group like AA, or perhaps even by hearing how others pray. If your main character is a priest, he may notice that p-zombies confess their sins in a different way than normal humans.