Well, as far as dementia/senility, I think that we could rule out just about anyone over the age of 70. But that would be ageism. So give them a mental status exam, it checks for short term memory loss and a few other items. If they are
perfect, then they can move on. (keep in mind that it is very rare for anyone over 70 to get 100%, and that is why I made the anti-aged remark
)
Otherwise, symptomatic schizophrenia, daily life effecting narcissism (hmmmm...), prior attempt at suicide...... anything that would endanger him/herself or those around. Anything that could make them perceive a nonexistent world. That level of difficulty.
He wants many things, he states that he will do many things; better than anyone else could, believe me. ....but he seems to have very little idea on how to make them happen. Reagan did it, but TRIPLED our debt. (sorry for another easy Reagan dig).
http://www.bbc.com/news/business-37013670
..."According to the nonpartisan Tax Policy Center, Mr Trump's plan would reduce the amount of income the government takes in by $9.5tn over the next decade. Mrs Clinton's plan would add $1.1tn in revenue over the next 10 years....."
PS - I'm not bashing Reagan. Just his inane policies, along with any who would doom us to relive them.