By American standards, anything that is less than 10 carats is not accepted as genuine gold. Therefore, it is important to check anything that's sold to you as genuine gold for markings. Usually, authentic pieces will bear the marking of 10k, 14k, 18k, 22k, or 24k. It alway helps to have a magnifying glass on hand to see the small markings.
It's easy however, for counterfeiters to mark fake gold as real, so further testing should always be used. Check for discoloration and signs of a base metal where the gold coloring has worn off. Fakes will always have a cheap base metal and a thin layer of gold.
There are common tests you can perform on your own, but they may not be foolproof. The old wive's tale about biting down on gold metal to test it is true. If your teeth leave small bite marks, you have genuine gold. Another method is to use a strong magnet, and if the jewelry is lifted up, it means you don't have real gold on your hands.
The most foolproof way to test your gold is to use the acidity test. Make a small pinprick mark on your gold in an invisible spot and drop nitric acid on top of the exposed pinprick. If your gold is fake, the color of the acid will instantly turn green.