Oh, come on. I have to say it.
If you go to a head shop, have head shop dude pull out the cheapest, the one for the most you're willing to spend, and something in the middle. Take a new penny, a new nickel, and a new quarter. Go online and find out how much each weighs. Put each coin on each scale and see if any is more accurate than the others. Also know if you want to go to tenths or hundredths of grams or ?.