If some things are computers and some aren't, then there has to be some rule differentiating the two, which gives a complexity penalty. Thus by occam's razor everything is likely a computer. Does this argument work?

Well, you can turn some unusual things into computers - billiard tables, model trains and the game of life are all turning-complete.

But I do have a good understanding of what a computer is. Perhaps my occam's razor based prior is that everything is a computer, but then I observe that I can't use most objects to compute anything, so I update and conclude that most things aren't computers.

Similarly, I can observe that most things do not have emotions, or preferences, or agency, or self-awareness. I can put a mirror in front of an animal and conclude that most animals aren't self-aware.

But is there any test I can perform to determine whether something experiences qualia?

Maybe, because humans often lie or have inaccurate self-knowledge.

Ok, suppose neuroscientists find a person who has not preferences for solid scientific reasons - perhaps the prefrontal cortex has been lesioned, or maybe they have no dopamine in their brain. Should you care about this person?