That's funny, it always seemed to work like that until I finished flying a couple of months ago?? Maybe it's changed since then... More to the point, at what point did I say that 'assuming the check-in and boarding staff do their jobs right'? Everybody who works in the airport is responsible for safety and security and passengers have often been caught out by being seen staggering towards the aircraft. Some check-in staff put comments in to have the pax checked at the gate if they believe they have been drinking (at least I did when I was ground staff in common with my colleagues) and the boarding team could often smell it too. Let's also not forget that the passenger has to pass through security en-route the the aircraft and can also be spotted there.
If all that fails, have you ever seen how close the cabin crew get to the passengers when they board an aircraft?! Alcohol is a pretty strong smell and passengers under the influence can fairly easily be detected here. Either way, the system tends to work although I do agree that as ground staff you do not get enough time to assess as perhaps you may in the past. I'm not suggesting it's the airline's agents who are solely responsible, but if everybody is aware (as they usually are) the problem can often be solved before the aircraft doors are closed.
FJ2k