*Check each piece with a magnet
*Grind the surface of questionable pieces. Steel is often chromed to prevent corrosion and will resemble brass; Die cast is often coated with a yellow coating which also resembles brass.
Yellow brass is the most common brass you will come across. It is used in belt buckles, plumbing, plaques, jewelry, marine hardware, musical instruments, and other things that call for an anticorrosive material. It can be painted, tarnished, or chromed so always grind the material to see its true colors. It is completely non-magnetic (a magnet should not stick).