A cheap fridge magnet is a quick first pass: steel and iron stick; aluminium, copper and brass generally don’t. Don’t rely on colour alone, though, and keep painted or oily pieces separate until you know your yard accepts them. I usually remove plastic, rubber and any sealed containers first, and I keep anything that looks like a battery or gas cylinder out of the scrap pile. The biggest practical issue I’ve run into is mixed bags—clean, sorted metal is much easier to recycle than a rusty jumble.