Which sensory system is ?

Well, all of them can be used to some degree. When physicians do a preliminary neurology evaluation, they use visual primarily to evaluate brain function and somatosensory primarily to evaluate nerve function, but either can be used for both to some degree. Olfactory would be a good one for brain function because the olfactory nerve is not a nerve per se, but an extension of the brain itself. I don't think any testing involving olfactory is taught in med school, though. Auditory would be largely useless because most people over the age maybe 5 or so have various auditory deficits due to damage to hair cells in the inner ear caused by loud noises. I suppose taste could be employed, but I suspect that patients might become rather unruly if they were asked to sample a variety of noxiously flavored liquids, for example.