Density. Gold is denser than virtually anything else, so by measuring the weight of the item, and its volume, you can determine the density. Pure gold weighs 19.3 grams per millilitre. The easiest way to measure the volume is by immersion in water, as found by Archimedes.
Note this only works for items that are solid metal. Gems or other things set in it will make the overall density lower, as would a hollow piece.