I get a simple dementia screening with my annual physical. Name the months of the year backwards, repeat a simple phrase with names and places at the end of the test that was given in the beginning, etc. While I'm confident in my age appropriate mental facilities, it has occurred to me that if I started to lose some of my abilities, would I be competent to recognize it.
We can assume competency based on media optics, but when Trump confused Haley with Pelosi and had delusions of winning the 2020 Presidential election, do we really know who has lost the most marbles, if any. There are probably people in Bellevue who think they are king of England.