Well if I remember right as a kid my dad had a 1936 Stinson and he would lean engine to quit first and then hit kill switch. If the switch was bad he wouldn't know it because the leaning would kill the engine first.
During the run up he should have checked the mags then, switch from both to l then R and back to both. A drop in RPM is an indication that the switch is working, however it could work during the runup but because of vibration causing contact still fail when the engine is not running.