SRS...Crew have specific training and capabilities to determine this themselves in the event that someone may be about to or is causing a disturbance onboard an aircraft as a result of being under the influence (note how I dont say "drunk"). We are trained to notice when passengers are about to go or be over the limit or behave irratically.
Now unfortunately the off side to this is that we must then deny access to alcohol for these customers and it is very difficult to explain to these people why we are not allowing them to have further alcohol without suggesting that they are drunk, it can be done but takes some major diplomacy skills and also depends on the passenger involved. Now Im not medically qualified but I do feel I am qualified to tell when someone is "under the influence"!!! I also know that should the poo hit the fan (from a legal perspective) that I am covered by the policies of my company and fully indemnified as I was simply carrying out a task asked of me as per my contract.
Now, I wasnt on this Scot Airways flight nor do I really know any of the details so I cannot comment on this but I assure you that I have dealt with many situations (seems to happen to me at least twice a month these days) of similiar nature and I must admit, what scares me most is the mix of reactions we get off other passengers in the immiediate area as they witness it regardless of how descreet it is carried out.