The recently released first part of an Air Clips documentary of a Lufthansa Cargo MD-11 FRA-NBO flight shows in some detail the pilot carrying out the preflight visual inspection of the aircraft (from ~ 6:20 min). As in many other pilots' accounts of these 'walkarounds', the captain states one important aspect is the condition of the engine, including e.g. fan blades, intakes, checks for any foreign objects.
I understand that a major point of this visual inspection is to have the pilot-in-command getting a full picture of the aircraft condition before assuming the final responsibility for the flight. Obviously, the PIC cannot inspect a center tail-mounted engine like on the MD-11 by himself during the walkaround – so when or how is that inspection done for the center engine? I imagine that an inspection of the center engine is just as important as for the wing-mounted ones, prior to each takeoff.