someone already mentioned it, but here’s the simplest form of NYS law which allows the sale of alcohol:
-valid ID proving an age of 21+ (a fake ID looking real is NOT valid; you using your brother’s real ID is not valid; your expired “real” ID is not valid)
-must appear sober (if there is even a SMELL of alcohol, refuse the sale)
-only during “drinking hours” (8am-3am during the week, noon-3am on sundays)
of course, you don’t know for sure if they’re drunk because you don’t have a breathalyzer. You can always have fun with the guy and ask, “What was Mickey Mouse, a cat or a dog?” If he has to think more than 5 seconds, he’s drunk. 
IMO: drinking age should be back down to 18. You can vote and die for your country when your 18-20, why shouldn’t you be able to drink?