Originally Posted by paul_c5x4
The United States Mint gives the weight for various coins. A cent =2.5g, a nickel=5g - A pile of these should be more than accurate enough to determine the accuracy of your scales.
Other countries will have a government site where one can find the weights of local currencies.
I think it is a best way to find out accuracy of your scale. But most of companies now offering certified scale with accuracy and you can easily adjust that scale Whenever you need.