When it comes to measuring gold content the most accurate and widely favored method is the fire assay. In this method someone scraped off a very small but from the gold object. This scraping is then wrapped in lead foil and silver is added to it. This combination is put into an extremely hot oven or furnace. The heat is so intense that it actually dissolves the silver. What is left is pure gold. This precious metal is them weighed.
Coming in second to the Fire Assay in accuracy in measuring gold is Inductively Coupled Plasma Spectrometry. There is no furnace used in this method. Instead, a small sampling of a gold object is dissolved in acid and the resulting substance is analyzed in an Inductively Coupled Plasma Spectrometer. This instrument is rather costly. So not only is this method not as accurate, it costs quite a lot more than the Fire Assay. One advantage that Inductively Coupled Plasma Spectrometry does have over Fire Assay is that it can measure the other metals that are alloyed with the gold.
The good news for you, however, is that you don’t have to do any of these things. When you want to sell gold, it is not up to you to test for the gold’s purity. If you sell it to an online gold buyer they will send you what you need and then you can leave the testing up to them. They will tell you how much your precious metal is worth and give you cash for gold and you won’t have to lift a finger.