The anwer to the original question is found in most Flight Operations Manuals (FOM).
Somewhere in that manual - probably under the heading of "Duties and Responsibilities of the Commander" - you will find this statement...
"The commander must satisify himself that the aircraft is airworthy"
Most captains that I know, including myself, would agree that to fullfill that requirement, means that someone who will fly IN that aircraft, should do some form of exterior inspection.