How did Archimedes find out if the crown was made of gold or cheaper materials?

He took the weight of the crown and then used water displacement to determine the volume. He could then deduce the density of the crown. Various types of metals have different densities. Density is the mass to volume ratio. Or in better terms, how tightly the molecules are packed together in a substance. If the density of the crown was different than that of any other piece of gold, it was not gold. The bath make him weight then die.