Look for color changes. If gold changes to black or green or leaves color on your skin it's low-quality or fake gold.
Weigh the gold in your hand to make sure that it is heavy. If the object feels too light, it may be fool's gold. Gold is heavier than most other metals that are the same size.
Try the bite test for gold coins. Bite down on the coin. If it doesn't get at least a little indent from your teeth, then it's fake.
Check to see if there is a stamp on the piece stating the carat of the piece. In some countries, such as the United States, all gold has to be earmarked with the karat. If the piece doesn't have this mark, then it's fake.
Look for ragged edges. Ragged edges on gold bands often signify cheap material that the jeweler can't smooth out properly. Since gold is a soft metal, it's very easy to get completely smooth.