If I remember rightly - it was a long time ago - when we did C of A air tests after major maintenance (VC10, 707 and 747), the flight engineer went outside with a headset and long lead while we checked the flight controls. He checked each of the control surfaces positions (ailerons and spoilers, elevators, rudder and stabilser) and coordinated his observations with the pilot doing the checks on the flight deck.
It pays to be thorough!