Check for a hallmark or stamp such as 10K, 14K, 18K, 22K, 24K, or 417, 585, 750, 916, 999
Look for signs of discoloration, especially where the piece rubs against skin or other surfaces
Use a magnet; real gold is not magnetic
Compare the weight to a similar-sized piece; real gold feels heavier than many fakes
Rub the jewelry on unglazed ceramic; real gold usually leaves a gold streak, while fake gold may leave a black streak
Apply a drop of vinegar; real gold should not change color
Use a gold testing kit with acid tests
Check the skin after wearing; real gold should not usually cause green or black marks
Inspect for wear on edges and clasps; plated jewelry often shows a different metal underneath
Have it tested by a jeweler with professional equipment such as an XRF analyzer