<<And what constitutes "potentially incapable"? How can you tell who is "potentially incapable"?>>
In the context of this thread, if a pilot visited me in ATC to "check out", or whatever it's called now, and was obviously the worse for drink/ drugs I would take every action I could to prevent him from flying.
I had close dealings with light aircraft pilots early in my career and whilst the vast majority are safe and behave very professionally, there are those who shouldn't be allowed on an aerodrome, let alone on an aeroplane.