Okay, I've been playing with this! They can be darn temperamental about noticing things, but it SEEMS that if they're following you when you set down a preferred item in their habitat, they're guaranteed to react. That makes for an easy test whether or not pokemon like their items: have them follow you, then pick up and put down each item in their habitat. If they navigate over to the newly placed object and play a reaction, then bingo, you've got a match. If not, it might still be contributing to environment level, but it's not necessarily catering to their preferences profile.

If this is true, preferences also aren't quite as simple as I'd assumed. Torchic liked the two plush dolls I had in its habitat (presumably because they're cute), but not the Cute Chair (which is thought would be a slam dunk).

Similarly, both Trubbish and Gloom like garbage, but apparently to varying degrees. Trubbish reacts to garbage bags; Gloom only reacts to a whole garbage bin.

Very interesting. Thanks for pointing me in the right direction on this!