Sorry to be so blunt, but...
Indeed, the toughest part of trying to help someone help themselves is in assessing their capabilities. Most people tend to overestimate their capabilities, but asking if someone is capable of certain procedures may be helpful. Many times, it seems possible to glean some insight or information based upon the nature of the persons’ posting or response…other than that, it’s pretty much a crap shoot.
Please do continue with your helpful ‘elaborate diagnostic strategy’ when it is apparent that it will be time well spent & not wasted upon those that may not be able to follow you.