Archimedes' principle
We do not actually know if Archimedes used exactly this method. An alternative is to use a scale. On one side put the object to be tested (e.g. the crown). On the other side put gold of equal weight. Immerse the scales. The gold will go down, and the crown up (if it is not gold). That is because, being less dense than gold, it occupies a larger volume and receives more buoyancy.