Methods for testing gold include acid tests, electronic gold testers and X-ray fluorescence spectrometers. Some of these tests are also able to determine the purity of gold.
The acid test uses a simple kit to determine if the gold is real or not and the karats. The kit typically includes several acid solutions and a black touchstone. The gold is scratched on the touchstone to leave a streak. Another similar piece of verified gold is scratched in the same way and the streaks compared. A drop of acid solution is poured on both streaks and the chemical reactions compared. This is the cheapest way to test gold.
Electronic gold testers, which are typically used by jewelers, provide reasonably accurate results when used properly. The tester determines the golds electrical conductivity. A special gel is used to create a circuit between the gold and the device. The results tell the tester whether the gold is real or not as well as its quality.
Lastly, XRF devices are considered to be the most accurate method for testing gold. The device excites the materials atoms into a higher energy state. The atoms release radiation as they return to their ground state which is detected and read by the XRF. The test determines the material the item is made of and its quality.