Magnet. Steel is magnetic; aluminum is not. Copper is red-ish; brass is yellow-ish.

A magnet is a very basic tool that every scrap metal buyer has AT ALL TIMES. My first month, I bought a 40# roll of copper-coated steel wire as copper wire. I felt real dumb until a scrap yard owner told me an employee once bought 5,000# the same way. Then I didn't feel so bad.