Fake gold can be produced by using another cheaper metal and coating the surface with real gold. This can be easily detected by weighing the object as the density of different metals are different.
For example, gold has a density of gold is 19.3g/cm3 while that of steel is 11.4g/cm3.
Therefore, a piece of real gold measuring 10cm x 10cm x 10cm would weigh 19.3kg while steel of similar size would only weigh 11.4kg. The standard gold bar held by central banks weighs 400-ounce (12.4 kg) and must be stored in recognized and secure gold bullion vaults to maintain their quality status and ensure maximum resale prices.
If the gold is of an irregular shape, the volume can be found be finding the displacement of water in a graduated cylinder.
The issue gets a bit more problematic when a fake material that has a close density to gold is used.
Uranium has a density of 19.1g/cm3 and would fit this criteria. However, it is radioactive so it is not that suitable.
Another metal is tungsten which has a density of 19.25g/cm3, almost similar to gold. Thus, a gold-coated tungsten would pass the density test with ease. However, tungsten is extremely brittle and has the highest known melting point of any non-alloyed metal at 3422 degrees Celsius, which makes it difficult to work with.
Nevertheless, gold-coated tungsten is being sold openly, one example is this China company, China Tungsten Online.
One author even suggests that fake tungsten gold is being widely circulated in the market, sometimes deliberately.
Short of cutting up a gold bar to see whether it is tungsten at its core, other ways of testing for gold fakes would be to use sound (sound travels at twice the speed in tungsten compared to gold), electrical resistance or thermal conductivity.