True, but one-sided methods, like spy satellites, aren't full proof and sometimes raise false alarms.
We came very close to nuclear war once when Russian computer mis-interpreted spy satellite imagery and falsely reported that the US had launched a single nuke. Fortunately, the Soviet officer incharge decided to ignore his orders to report any warnings because he correctly judged it to be a false alarm. If he had been a brow beaten automaton trained to strictly following his orders, and relayed the launch to his superiors, they might have decided to immediately counter-attack, starting nuclear armageddon.
https://en.wikipedia.org/wiki/1983_Soviet_nuclear_false_alarm_incident