Since robots don't readily suffer illusions ...
As a failing peculiar to animate visual systems, visual illusions might be used to distinguish humans from "computer bots", or any other artificial intelligence empowered with a visual capacity. Any such entity is unlikely to suffer the same illusions as our own, unless, of course, it has been specifically engineered to do so. This approach inverts, and complements, the logic of the Turing test: not requiring evidence of an intelligent capacity equivalent to that of human beings, but rather that of a characteristic human failing.