If the gold jewelry does not have a stamp or marking indicating its karat, there is chemical testing that can determine the karat or purity of the gold. A jeweler can perform this testing, and kits are available for home use that are fairly accurate.
To test gold for purity, the jeweler scratches the gold against a testing slab, leaving gold residue on the stone. The residue then has acid applied to it, which causes a chemical reaction that can reveal the purity of the gold. Home testing kits follow a similar procedure, and they also provide information regarding how to read the results properly.