You go out to the aircraft when they've just done the pre-flight on their own for the first time, and ask them if they checked the water in the radiator.
Or you can give them a fright by REALLY meeting them at the aircraft in ten minutes, instead of the usual thirty...
And ask them to go into the hangar and ask the engineers for a long weight, left handed screwdrivers, half a pound of mag drops etc etc etc