It is important to test gold coins before you buy them. Expensive gold coins are commonly forged by counterfeiters in order to cheat collectors. Most of these fakes are fairly easy to detect if you know what to look for. If you suspect that a gold coin might be a fake, take it to a professional coin dealer for an assessment.
Look up the gold coin's specifications. All gold coins of the same year, type and denomination are made exactly the same way.
Measure the coin. The diameter should be exactly what is listed for that coin. If it is even 1 millimetre off, it is probably not genuine.
Weigh the coin on a very sensitive scale. This should ideally be a coin scale, but a jewellery scale or postage scale will do. Most fakes weigh less than genuine coins.
Closely examine the surface. Fake gold coins often appear too dull or too bright. Place it next to a gold coin you know is genuine. The colour of a fake coin will be different. Counterfeit coins often lack detail and appear grainy since many are cast from moulds of real coins. Examine the edge. If you see a seam, then it is probably two cast sides glued together.
Look through a magnifying glass at the coin's date and mint mark. These details are often altered on genuine coins to make them appear to be a rarer date and mint mark combination. Details can be added or removed. Look for scratch marks, glue spots and changes in colour.
Drop the coin on a hard surface. Genuine ones make a ringing sound when dropped. Fakes make a thudding sound. Do not perform this test if the coin is in pristine condition or especially valuable.
Have a professional coin dealer examine any gold coin you consider purchasing. He should be able to tell instantly if it's a fake. If the seller refuses to allow a professional coin dealer to look at it, be very suspicious.
Never bite a gold coin to determine whether it is real. This tells you nothing about the authenticity of the coin. It also can damage the coin or your teeth.