Well. number one, the stock pulley is easily identifiable since it is not easy to remove and has a three arm octopus that attaches to the engine. All aftermarket pulleys usually have six bolts that attach the pulley to an adaptor, which is held to the crank with three bolts. Bottom line being, if you can see six bolts on the front face of the pulley, it isn't stock. Typical is a six pound lower. Which is six pounds over the stock eight pounds. Stock pulley is 7.5 inches at the surface the belt rides on. Every half inch is two pounds over. So an eight inch is two pounds, 8.5 is four, 9.0 is six. If it's over that, DUCK!!!!
I feel I am safe to say, add 1/4 inch for outside diameter. So that stock 7.5 would be 7.75. Etc.