By American standards, fake gold is anything less than 10 karats. If you're wondering if your gold is real, the most reliable way to find out is to take it to a certified jeweler and have it tested. If you want to check for yourself, use these instructions to make that determination.
Hold a magnet up to the item
Gold isn't a magnetic metal, so if it pulls towards or sticks to the magnet, it's fake. However, just because it does not react to the magnet does not mean it's real, as non-magnetic metals are used in counterfeit pieces as well.
Inspect the piece for official markings
A stamp indicates fineness (1-999 or 0.1-0.999) or karat (10K, 14K, 18K, 22K or 24K). A magnifying glass makes this easier to see. An older piece might not have a visible marking due to wear. Counterfeit pieces often have a marking that appears authentic; more testing may be needed either way.
Look for noticeable discoloration
It's important to check for discoloration in areas that face constant friction, typically around the edges. If the gold seems to be wearing off and showing a different metal beneath, you probably have a piece that is only gold plated.