Pinkman pointed out that your colleagues are not always going to be able to call out a drunken pilot as they may be drunk also.
Obviously, that is true but no matter what provisions are put in place somebody will always find a way around whatever system is in place unless we start being tested at sign on and every half hour thereafter.
As far as being infallible, the human race sucks!
In Oz, we have a pretty good system where those that know they have a problem and act, can get help without losing their livelihood or trying to hide the problem (which is where the real problems start)
It does occur to me that in my 17.5 years of flying, I've been tested, for alcohol only, on two occasions both of which were at outports, up to 4 hours after sign on.
I'd like to see a bit more random testing, I don't think it would be too burdensome on the industry or the regulator.