Gold has historically been considered a precious metal, and one of its primary uses has been to make jewelry and decorative objects. Brass can have a similar look to gold, but it is substantially cheaper. A magnet can often determine the difference between gold or brass, as brass will be attracted and gold will not. But when in doubt, a simple acid test can confirm whether an object is gold.
Use a magnet. Brass is magnetic, so a magnet will attach to it. Gold is not magnetic, although gold may be alloyed with other metals that are magnetic. Generally gold--even low-quality gold--will not be attracted to a magnet. If the object is attracted to the magnet, it is very likely not gold.
Look for markings on the piece. Often, pieces that are gold will be labeled 10K, 14K, 18K or 24K. The K refers to the karat, or the purity of the gold. This is a good indicator that a piece may be gold. Brass items may have markings saying Brass.
Use an acid test. There are various acid tests on the market for testing gold. If you still are not sure after testing with a magnet and looking for a marking, make a small scratch in a protected area that is not readily visible and apply a drop of the testing acid you select. For example, with Schwerter's testing acid, brass items will turn dark brown, while gold will have no reaction.
There may be mailing restrictions on acid kits depending on the state you live in.
Do not try an acid test on rare coins--doing so may diminish their value.
Take care when using the acids, as they are highly toxic.
If you use an acid kit, make sure you read the directions carefully, as the colors may be different depending on the acid.