This is a very difficult subject, in flight recently I was told that the cabin crew suspected one of their number had been drinking but was acting normally, I had not noticed anything unusual about this person when we checked in for the flight and getting the information just as we are on the final approach resulted in me deciding to go ahead and land ASAP.
I asked for the person to come to the cockpit after the Pax has left the aircraft and their breath did smell suspicious, I put this matter in the hands of the company management.