I use a postage scale--spring loaded, not digital. I make sure to zero it from time to time. It goes up to 5 pounds with a swinging needle indicator. I just tested it with a 2 pound weight, and it's quite close.
I use a digital scale and it seems pretty accurate based on the number of servings of things I weigh and the fact that it matches up. Example, can of pumpkin had 3.5 servings based on label and surprise, I came out with just a little more than 3.5 servings after finishing the can.