On your way to the tackle shop, stop at a grocery store. Buy 5# of sugar and 10# of flour. Each will weigh a tiny bit more than the advertised weight due to packaging and the maker being sure the customer can't accuse them of shorting the customer. You can now test the scale the salesperson swears is accurate at 5, 10, and 15 pounds. Look more for a repeatable % error than absolute accuracy.
the one I bought showed 5.3, 10.6 and 15.9# at the three weights. Others tested that visit to the tackle shop were all showing much more than the actual weight.