Most of the budget scales I've seen are accurate to <0.1g. If in doubt, grab a cheap set of calibration weights for $20, I have 2 sets from China and both are accurate to <0.01g on all the weights from 1g up to 200g.

Obviously if you have the money, you can buy actual certified ASTM weights, but they are insanely expensive.

You can calibrate the scale with coins as well. They have a pretty well defined mass.