how to tell the difference?

Also - "Cast" (anything - Iron, Aluminum etc) means Breakable. So if it can be bent, it is not cast. Copper is "red-ish" and Brass is "Yellow-ish". For stainless steel, you'll need experience and study since stainless steel can be Ferrous or Nonferrous. Stainless steel is often confused with Aluminum. A simple test is to use a grinder - Aluminum will not throw sparks; stainless steel will. Extrusion (a form of aluminum) is manufactured thicker than sheet aluminum (a simple test I use is if I can tear it - it is Sheet. If it just bends and breaks without tearing, it's Extrusion. If the yard disputes it, "Oh well".)