Most gold jewelry is is stamped with a hallmark noting the jewelry's karat weight, such as 10k, 14k, etc. However, not having the hallmark doesn't always mean that the gold is fake. Taking the piece to a reputable jeweler is the only foolproof way to verify whether or not a piece of jewelry is real gold, but there are some tricks you can try at home that can be accurate if done correctly.
Make a light scratch on the piece of jewelry with a small file. Pick a spot on the jewelry that will not be noticeable so you can still have the option to wear it if you should choose. Using a dropper, apply a drop of nitric acid on the scratch you made on the jewelry. If nothing happens, the piece is likely real gold. If you see green, you are likely dealing with a base metal or a gold-plated piece of jewelry. If what you see is a milky substance, then you likely have a piece of jewelry that is gold over sterling silver.
Apply liquid foundation in a small area on your arm. Layer it with powder makeup. Rub the jewelry against the spot. If you see a mark, then likely you have a real gold piece of jewelry. If you see nothing, then the piece is likely fake.
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Drop the piece of jewelry in question into a jug almost filled with water. Gold should be heavy. If it sinks, that is a good sign that it could be real gold. If the piece floats, then it is probably fake.
Take your piece of jewelry to a reputable jewelry dealer. They are equipped with testing kits that can determine whether or not the piece is real gold. Some jewelry stores will charge you a small fee for this service, but it is foolproof if that is what you want. Unfortunately, even a jeweler is not going to be able to tell if a piece of jewelry is real gold just by looking at it.