Difficult situation, as always.
Risky and it does depend on the personality of the individual(s) concerned but you could try having a quiet word with them and voicing your concerns. If that doesn't work you could give them a gentle warning that you're not prepared to condone their actions any longer.
Failing any measure of positive change from them, you could always resort to the last option and get on the blower to the local police and tell them you know of a working pilot who you believe to be under the influence. Regardless of whether they're actually flying or not, the Railways & Transport Safety Act 2003 Part 5 also applies to Flight Crew when they turn up for duty (ancillary functions), with an alcohol limit of 20mg per 100 ml blood as opposed to driving which is 80mg per 100ml blood. That would mean prosecution for the individual and I'm not going to go anywhere near the Health & Safety laws that would be broken by the Club.
However, this would not be the preferred choice as improvement is always preferable to enforcement but the chances are that people who insist on flying whilst impaired through drink or drugs are one day going to kill themselves, their passengers or someone else on the ground.
As for doctors / police being qualifed to say that someone is drunk I believe that also to be correct, the criteria being:
Glazed eyes
Unsteady gait
Slurred speech
Breath smelling of alcohol
Realistically, though, most people can tell whether someone is ratted or severely hung over and whilst they may not be able to legally enforce it they can bring it to the attention of someone who can.
Saying all that, I don't think the problem is prevalent in GA..............or am I missing something?