Caveat: I'm only a Class 2 medical graded pilot. However, I have had exposure and experience of many industry approaches to D&A testing. Most just operate on the basis of suspicion. If the management think you are woozy they will test you breath or urine. Some consider themselves to be ahead of the game and have a randomised approach to this.
There is a significant exception: the railway industry. Not just the operators, but maintenance, designers, supervisors, and trainers. It's broad and significant.
The test is NOT just blood alcohol at the point and the time. Before employment you have to voluntarily have a test of you urine: and they look at the protein levels associated with your generic alcohol usage pattern. Oh yes. And history of drug use going back several months.
I'm more concerned about my rail industry tests than I am about my Class 2 medical. That is very amateur by comparison. (Yes I have done a Class 1, that was years ago).