To my knowledge there is no requirement that the public be honestly informed of a Presidential candidate's health. Curiously, we seem to worry more about their tax records, which may be misleading; while history is full of examples of candidates who themselves have, or whose political "handlers" have, sought to obfuscate health-related issues.
In many cases, the things that were kept as secrets were not really debilitating or disqualifying, except from the standpoint of public image - they could have made the candidate look "weak." Examples include FDR's crippling polio or JFK's back problems. Eisenhower's heart problem was a somewhat different matter - concealment was justified as a desire not to "alarm" the public. A heart attack at the time was a potentially serious problem which might be less so today because of medical advances. If a President should die of a heart attack, or the (sadly) statistically more likely bullet, there's an app for that - it's called "The Vice President." It's usually pretty clear when the Veep should step up.
Today, we are in a different, and potentially more dangerous phase in which we must consider disorders that are less black-and-white. They're more in a gray area - the brain. In my view, the dissembling that surrounded Ronald Reagan's Alzheimer's was of a different order of magnitude than other concealment efforts, because in such cases, serious errors can occur before we are aware there is a problem. When the border between competent and incompetent is fuzzy, concealment of the condition is more likely to delay the necessary decision that it's time for the Vice President to take the helm.
The effects of certain diseases often manifest themselves in subtle ways before they become outwardly and publicly evident. I think of the late Muhammad Ali, whose undiagnosed ailment of early Parkinson's is now being suggested as the cause of some of his surprising losses in fights he was fully expected to win. And I think of Reagan, who to my recollection didn't exhibit noticeable symptoms of anything serious during his candidacy or early Presidency, but whose later miscues or struggles to find a word might have alerted us to think it was not just "old age," but maybe something more serious.
So it seems that without any hard-and-fast requirement that a Presidential candidate's physical and mental health be formally evaluated (legislation that, like sensible gun regulation, is unlikely to be achieved in our lifetime), voters will need to draw their own conclusions as they have always done about a candidate's fitness for office. Not just his/her political views, government experience or lack thereof, and how fun it would be to have a beer with him/her, nor even whether he looks like he's about to have a heart attack, but also clues as to his mental state and real capacity for decisions and logical thinking.
What clues? I'm old enough to have seen friends and colleagues pass from "normalcy" to dementia or Alzheimer's. In my inexpert observation, early signs often include grumpiness, belligerence, an insistence on having one's way, a seeming childishness. Or, the website alz.org offers this brief but presumably more scientific summary of symptoms (my highlights):
In early stages of the disease, people may experience personality changes such as irritability, anxiety or depression. In later stages, other symptoms may occur, including sleep disturbances; agitation (physical or verbal outbursts, general emotional distress, restlessness, pacing, shredding paper or tissues, yelling); delusions (firmly held belief in things that are not real); or hallucinations (seeing, hearing or feeling things that are not there).
As voters, we have always had to make our own judgments of fitness based on insufficient information. Now there's a new dimension to the job. I'm just sayin'...