Perhaps they could video the cockpit (and the fuselage for that matter) and destroy the footage once the plane has safely landed. There could be streaming capability to the ground and if the feed is accessed, the pilots and crew receive a notification. Any unauthorized breach would be detected immediately. In the case of Germanwings, ground control would have been able to see what's going on once they detected the loss of altitude. It stifles me that in 2015, a young troubled copilot can end 150 lives in a way that can easily be prevented with simple technology.
How is it any different than a ferry captain or a bus driver? There are many more fatal accidents from both of those occupations and neither has much of any monitoring.