So extend the causal chain a little. After a certain number of consequences-of-consequences, it can't be a trick of the eye, not in an integrated and causal world, you would have to have a fully convincing daylight hallucination or be faced with explaining a negative result.

Someone with anosognosia would just launch into a contrived explanation. So a positive result is as conclusive a clean bill of health as any analysis using human senses.