I think the best final defence against an intoxicated pilot showing up for work is the pilot sitting next to them. All the talk of 100 percent testing, training security screeners (really!) etc, is nonsense.
Where afety sensitive roles exist, such as pilots and air traffic controllers, random screening is needed at frequent enough intervals that people know it's a legitimate possibility, where people self declare a problem, they are screened randomly and often, where there's an incident they are screened (and educated that this will always happen after an incident) and the final defence will always be the guy or girl sitting next to you.
While I am aware that there are a few famous examples of both tech crew showing up intoxicated together, the cabin crew or other colleagues have called them out.