This is kind of tough to answer because overall medieval and Early Modern people were more open to the idea of visions and benign spiritual revelations than we might be now. For that reason it's next to impossible to say if the experience that a medieval individual identified as a divinely-inspired vision was in fact a hallucination or otherwise the fruit of a psychological disturbance. There was ample documentation in Christian scripture of individuals receiving divine visitations (being visited by angels, for instance) or hearing divine voices; for medieval thinkers these occurrences were still seen as within the realm of possibility, and as benign or desirable occurrences, not as inherently troubling. There was a lot of concern and interest in the idea of prophecy as a still-ongoing phenomenon, and a lot of intelligent people put stock in the idea of prophetic visions and divine voices. But sensory experiences without a material source were not unilaterally attributed to divine sources -- discernment was necessary because of the risk that visions and voices might be sent by the devil rather than God, or they might be incidents of mundane origin, stemming from physical illness or invented by the individual to lend credence to heresies and errors they came up with themselves in a mundane way. People who reported visions that no one else could attest to might just be liars or fools, rather than visionaries or madmen.

A comparable phenomenon might be the medieval and Early Modern tradition of prophetic dreams -- not every dream was a prophetic dream. So how do you tell when your dream or your waking vision was telling you something important and actionable, and when it wasn't? The content of the voices' statements, or the nature of the visual hallucination, might have made a difference -- if voices were telling you to sit down and pray, they were benign and you could do what they suggested without issue, but if voices were taunting you with lascivious words or telling you to do outrageous things, they could be readily categorized as devilish voices, not angelic. (Likewise with visions -- seeing the faces of saints or perceiving the foreskin of Christ was mystic, seeing monsters that no one else could see was devilish.) The motif of being taunted by visions and apparitions of malicious origin was also well-documented in Christian tradition by that point -- St. Anthony in the desert being bothered by devils, or Jesus being tempted in the desert by Satan -- and individuals might have understood hostile hallucinations that way, both their own and other people's. If someone's visions or disturbances were causing them distress, they were likely to be attributed to a disturbance and not a vision -- that disturbance might be understood as essentially physical (a humoral imbalance, for instance) or as essentially spiritual (being tormented by devils, or by the devil) or as a mixture of both. Medieval people weren't completely credulous -- they were aware of the risk of a psychologically disturbed person making unwise decisions, especially disturbed people in a position of power. A single person's report of their experience would be subject to outside scrutiny, especially if they otherwise presented disordered behaviors, and was as likely to be discounted as the ravings of a madwoman as the authentic account of a spiritual phenomenon. Even unusual behaviors that by themselves might be tolerated would provoke worry if the person presenting them was in a position where other people relied on their reason and judgment -- monarchs known to be "mad", for instance.

Authors have examined the experiences of many medieval mystics through a medical lens, attributing experiences that mystics identified as religious or spiritual experiences to conditions like schizophrenia, epilepsy, or migraine. The most obvious candidate for an individual who acted on the basis of "hearing voices" might be Joan of Arc. In modern terms, Joan experienced auditory and visual disturbances from early adolescence, in the form of voices, images, and bright lights. Joan herself understood these voices as the voices of St. Margaret, St. Catherine of Alexandria, and the archangel Michael speaking to her. The voices told her to take up arms and drive the English out of France:

If I were in a wood, I could easily hear the Voice which came to me. It seemed to me to come from lips I should reverence. I believe it was sent me from God. When I heard it for the third time, I recognized that it was the Voice of an Angel. This Voice has always guarded me well, and I have always understood it; it instructed me to be good and to go often to Church; it told me it was necessary for me to come into France. You ask me under what form this Voice appeared to me? You will hear no more of it from me this time. It said to me two or three times a week: 'You must go into France.' My father knew nothing of my going. The Voice said to me: 'Go into France!' I could stay no longer. It said to me: 'Go, raise the siege which is being made before the City of Orleans. Go!' it added, 'to Robert de Baudricourt, Captain of Vaucouleurs: he will furnish you with an escort to accompany you.' And I replied that I was but a poor girl, who knew nothing of riding or fighting. I went to my uncle and said that I wished to stay near him for a time. I remained there eight days. I said to him, 'I must go to Vaucouleurs.' He took me there. When I arrived, I recognized Robert de Baudricourt, although I had never seen him. I knew him, thanks to my Voice, which made me recognize him. I said to Robert, 'I must go into France!' Twice Robert refused to hear me, and repulsed me. The third time, he received me, and furnished me with men; the Voice had told me it would be thus. The Duke of Lorraine gave orders that I should be taken to him. I went there. I told him that I wished to go into France. The Duke asked me questions about his health; but I said of that I knew nothing. I spoke to him little of my journey. I told him he was to send his son with me, together with some people to conduct me to France, and that I would pray to God for his health. I had gone to him with a safe-conduct: from thence I returned to Vaucouleurs. From Vaucouleurs I departed, dressed as a man, armed with a sword given me by Robert de Baudricourt, but without other arms. I had with me a Knight, a Squire, and four servants, with whom I reached the town of Saint Urbain, where I slept in an Abbey. On the way, I passed through Auxerre, where I heard Mass in the principal Church. Thenceforward I often heard my Voices.

(In general, if you're interested in how people who experienced voices and then acted on them understood those voices, Joan's testimony while on trial is really interesting.) Joan went on to do what the voices told her, and to experience a number of other "revelations and apparitions" in the course of her military career until it culminated in her capture, her trial for heresy and cross-dressing, and her execution. So in a way, Joan was both believed and not believed, by different people at different times and for different reasons. I myself don't know what I think about Joan -- I think to discount her or any other medieval/EM vision-experiencers (Margery Kempe? Ignatius of Loyola?) as simply crazy or simply a modern-style schizophrenic without considering the religious and social context around visions and saintly voices is selling the situation short. Explaining historical phenomena in purely modern terms starts to look a lot like explaining them away when scholars do that -- people couldn't really have believed this stuff in earnest, of course, they must have been sick or stupid, and so on. But there's a ton worth examining around the sensory and physical aspects of disturbances that the people who experienced them took for granted as mystical, and how those relate to modern experiences of sensory disturbance.

You might also be interested in the situation of Charles VI of France, known as "Charles the Mad" -- like Joan, he's a 14th-15th century figure, not a 16th century one, but still an interesting case study. The most famous aspect of his condition was that he periodically believed himself to be made of glass, but he experienced other psychotic episodes and periods of disturbance that negatively impacted his rule and at times had a death toll. During his first documented psychotic episode, he killed four men in a frenzy while under the impression that he had been betrayed and was about to be assassinated. At the same time, this was precipitated by an assassination attempt on one of Charles' own intimates, so it wasn't what one might think of as a delusion rising up out of nowhere -- his actions were fueled by the realities of Charles' status and the contemporary political situation, and the king's family and advisors had good reason to fear that he might undertake other actions on the basis of his disturbed perceptions and cause political trouble. At the same time, "mad" kings and princes were vulnerable to the machinations and interventions of their family members, and their madness might be the pretext for removing them from their positions to be replaced by a more stable monarch.

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