Look for a hallmark or stamp such as 10K, 14K, 18K, 22K, 24K, or 417, 585, 750, 916, 999
Check for discoloration or wear that reveals a different metal underneath
Use a magnet; real gold is not magnetic
Compare the weight; gold feels heavy for its size
Rub it on unglazed ceramic; gold leaves a gold streak, not black
Test with vinegar; real gold will not react
Use a gold testing acid kit
Check for tarnish; gold does not tarnish easily
Inspect edges and worn areas for signs of plating
Have a jeweler test it with an electronic gold tester or XRF machine