The launch control officers probably watch cable and listen to the radio. They will not switch their keys until radio stations disappear, cable signals vanish, and NORAD has given clearance. They probably also rely on seismometers.
In the movie, The Forbin Project, the computer AI figured out a way to DETONATE a nuclear warhead INSIDE the silo, when humans tried to neutralize them by replacing them with fake detonators at regularly scheduled maintenance times...................