Most drug/alcohol users actually get caught that way. If you are watching a guy work it doesn't take long to figure out he is either stoned or just the dumbest guy on the planet. In either case, you should have him "seek other opportunities".
The only good thing about a drunk/druggie is you can fire him on the spot, a dumb guy needs to follow the process and that might be a long and winding road if you don't want to eat the unemployment and potential lawsuit.
