Mackaques love touchscreens, amd can be trained to do complex tasks on tablets placed in their enclosures.
You make a test where they need to press a button when and only when they see tiny monkeys that you can project on a screen or on their floor.
Then you give the drug at various concentrations and placebo, and see if there is a dose dependent increase in the monkeys indicaions of mini monkeys.
Then you can repeat the experiment after treatment with antipsychotic, if it prevents the mini monkey reporting, you already have a hint at the mechanism.
Any nonhuman primate lab can do this from 1-2 million dollars, should we start a go fund me?

Get one of them sign language moneys and see if they start asking for tiny bananas

Can't you just ask them? Like train them to show what they are seeing by pointing to one of multiple pictures.

I think you could watch pupil response, body temp, sweating, brain scans, etc and get a good idea.