Upvote:3
I'd go with hardness as Silver is soft but becomes much harder the more copper content is present. Take a known silver coin and a coin in question and setup an experiment where a jewelry hammer hits the coin with the same force. A pure silver coin should indent with a significantly lower force than the silver/copper coin. It's a pretty steep curve with a 10% copper impurity being almost twice as hard as a pure silver coin would be.
image credit here http://riograndeblog.com/metal-hardness-how-to-measure-it-and-why-it-matters-for-silver-alloys/
Upvote:4
The density of copper (~9 g/cc) is almost 20% less than that of silver (~10.5 g/cc). It wouldn't take a genius of Archimedes caliber to repeat the well known "Eureka!" experiment and test coins for precious metal content.
And the moment one person in a community starts discounting certain coins, you can be certain that others would get suspicious as well. It just takes one well educated merchant to start the rumours flying.