how to tell if something is made of Copper?
Copper the metal? There are only two metals that have a yellowish color, copper and gold. So if it's yellow, it's probably copper or a copper alloy, such as brass or bronze. .
Metallic copper has some very recognizable physical properties. There are plenty of reliable chemical tests for copper ions.
If we're talking about salts...burn it and see what colour the flame is
