I've read that in advanced alcoholism, the alcoholic becomes super-sensitive to booze and looks/acts drunk with only a small amount of alcohol ingested - I've also read that advanced alcoholic develop a tolerance and need more and more poured into their system before they get the buzz (and start looking drunk). So your mileage may vary.
I also think thequest is right in that the physical signs of alcoholism show up in places other than behavior. My alcoholic ex has been on the bottle for about 35 years - he can appear quite lucid in his speech, but the physical damage shows up in other ways, which a casual observer might not connect to alcohol. He has a "traveling tremor" that's pretty much constant now, and a characteristic lurching gait. Anyone who didn't know him might think he just walks sort of funny, or was in the early stages of Parkinson's disease.