You can tell whether parts are zinc (or if they are not plated, perhaps aluminium) in various ways, but usually I just look to see whether it is a die-casting. Iron and steel are not suited to die casting because of their high melting temperature, though they can be sand cast. Zinc is even easier to die cast than aluminium, and is stronger. Incidentally, for some reason Americans like to call zinc "pot metal".
Of course steel or iron, unless it is an austenitic variety of stainless, is magnetic and few other metals are, so you can usually detect steel and iron with a magnet.
