It is fairly simple to authenticate gold coins and gold bullion. Fake gold coins and gold bullion do not look, feel or sound like real gold. There are also significant physical differences that can be detected by measuring. A small percentage of fake gold coins are so convincing that only an expert can tell that they are not authentic. If you doubt the authenticity of a gold coin or gold bullion, you should consult a gold expert.
Measure the gold coin or gold bullion. All gold coins and bullion are made to exact size specifications. If it is off by even a millimetre then it is probably a fake. A 1 pz. gold American Eagle is exactly 32 mm (1.26 inches) in diameter. A UK Sovereign is exactly 22 mm (0.87 inches) in diameter (see Resources).
Weigh the gold coin or gold bullion. All gold coins and bullion are made to equal precise weight. If it is even slightly off, then it is probably a fake. A 28.4 g (1 oz) gold American Eagle weighs exactly 28.4 g (1 oz). A UK Sovereign weighs exactly 6.66 g (0.235 oz).
Examine the surfaces of the gold coin or gold bullion. Fake coins are often dull or overly shiny. Their colour can be more yellowish than gold. Designs of fake coins often lack detail. This is because they are often cast from moulds made from real coins. Compare the gold coin or gold bullion to an item you know to be authentic. If it appears different from the authentic coin, it is probably a fake.
Listen to the sound the gold coin or gold bullion makes when you drop it on a hard surface. A real coin will make a ringing sound; a fake coin will thud. Only perform this test if maintaining the quality of the coin's design is not an issue. Dropping a brilliant uncirculated coin will damage it and reduce its value.
Feel the gold coin or gold bullion. Fakes will often feel slippery, too smooth or have sharp edges. Close your eyes and feel a coin you know is real in one hand and the suspect coin in the other. If they feel different, then one is a fake.
If you are not certain whether you have a real or fake gold coin or gold bullion, it is a good idea to get a second opinion. Take it to a local coin shop or business that buys gold. If it is real gold, they will offer you cash for it. If it is fake they will tell you immediately.