I have always thought this was a stupid law. Let's say the same man goes to the liquor store and buys a 12 pack and drinks the whole thing and goes out and gets into an accident. The liquor store cashier isn't held responsible, because there's an assumption that the man will pace himself.
In your defense as a server you could say "he kept a little Chihuaha with him, so I assumed that he was giving most of the alcohol to the dog." It sounds crazy, but you could say "well, bartenders aren't authorized to search patrons...it's illegal, and immoral." I can see that shit going all the way to the Supreme court.