Hmmm!! Tom, with all the knowledge you seem to possess, I'm surprised you didn't mention the easiest test of all.
If you make a fast pass into a common "coin dip", almost any tarnished strike comes out rather nice looking...shiny yellow brass... because it was it's original color.
Dip a "copper" strike and all you will get is a strike with a copper color...because that was it's original color.