One could measure electric potentials / chemical concentrations in the nervous system. If experimenter shines a light on individual's eye and the amount of neural activity deviates from the average response distorted view of reality is detected - either blindness or hypersensitivity, depending on the sign of the deviation.
There are many techniques for measuring activity of the nervous system that differ in accuracy, invasiveness, costs. The technologies may come from neurology or other medical fields, but are often applied in cognitive science, cognitive psychology and even bleed into linguistics (in study language acquisition and speech disorders). Cognitive neuroscience is ... concerned with the scientific study of biological substrates underlying cognition, with a specific focus on the neural substrates of mental processes.
Traditional technique - that you often see in fictional portrayals of truth detectors - is electroencephalography (EEG) which "measures voltage fluctuations resulting from ionic current flows within the neurons of the brain".
Many recent experiments are based on functional Magnetic Resonance Imaging (fMRI) "that measures brain activity by detecting associated changes in blood flow".
Positron Emission Tomography (PET) "produces a three-dimensional image or picture of functional processes in the body [via detecting] pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule". PET use for neuroimaging "is based on an assumption that areas of high radioactivity are associated with brain activity. What is actually measured indirectly is the flow of blood to different parts of the brain, which is, in general, believed to be correlated, and has been measured using the tracer oxygen-15".
There are also other - e.g. measuring electric potentials of individual neurons, optogenetics: making the neurons photosensitive and then shining lasers on them to switch them on and off (optogenetics isn't on its own useful for measuring "processing of reality", but could be used for testing hypotheses about it, I guess).