Clashing known things with unknown.
Since the statement says, the brain creates the stimulus describing the world, it will be able to describe only, what it already knows, or capable to make up from fractions. It can be challenged. And if discrepancy is catched, it can be a base of proving, the experience is an illusion.
Everything we sort as fantasy in current time literature, is an imagination. If an imagination can be identified, it is a proof for illusion.
Or for the sci-fi Blade Runner employs a questionnaire, which serves as tool to decide whether the questionned person is a clone or
real original person.
Samples of indeterminated occourances
Topics, which would qualify in real world as active research field in science, but do not have results yet, outcomes are not trivial. A brain, which has not collected result of a research may create different outcomes on different parameters, or even on (thought to be) the same.
At some point, people on Earth supposedly realized that the Sun is not following the very same track day-to-day, but is has a slight change as it progresses in time for seasons. It can be called belief, but thinking that all days are the same is an illusion.
A fun example for people would be the mask joke for Prince John in Robin Hood: Men in Tights (1993). He had a wart on his head, but always placed to a different spot in every scene. Description said, he has a wart, but no detail on exactly where. Warts do not tend to change place, and due to lack of information, several makeshift instances can be generated.
A brain is attempting to apply patterns. If a single brain is generating illusions, they will always be plausible, straight-forward, doubtless. There will be no suprises, everything will follow on a calculatable direction (I do not mean necessary calm, but predictible). If you never meet anything, that you would mark as suprise, "never would have thought about that", it is a good sign to begin coming up with second thoughts.