You can differentiate real gold from fake gold by completing a density test, magnetic test, nitric acid test or by checking for a hallmark. A sure way to tell if gold is real or not is to take it to a professional appraiser.
One of the easiest ways to determine if gold is real or not is by doing a magnetization test. Real gold is not attracted to magnets. If the gold piece in question is attracted to the magnet, then it is mostly comprised of other metals and has only a gold plating on top that masks the main metal.
Another effective way to tell real gold from fake gold is by looking for the hallmark. A hallmark stamp includes the karat weight of the jewelry. These markings are usually visible on the clasp on necklaces and bracelets or on the inside band of a ring. Karats are described using the letter K; if the number 24 or 14 accompanies the K, then the gold is real. In 14K gold, just over half the material is gold; fake gold is anything less than 10K.
In addition, fake gold jewelry tends to have visible tarnishing and eventually shows wear on the plating.