How to verify or detect the percentage(carat)of purity in the gold by general/scientific method?

Answer 1

There is X-ray fluorescence instrumentation available to test the karat measurement of precious metals and finished jewelry. These machines allow a karat gold measurement without damaging fine jewelry.Some can also accurately determine the element composition of gold jewelry including gold, platinum, silver, paladium, rhodium and related alloys.

Answer 2

There is a simple way, by getting its mass and volume, then determine its density and compare it with pure gold density.