http://commoditybullmarket.blogspot.com/2009/04/how-to-detect-counterfeit-gold-coins.html

If you have the facilities to melt a bar, you can adulterate the alloy to reduce the gold content. For example, if you start with a tungsten slug and wrap it in aluminum you can get the density of a core correct, and then surround it with gold you can get a bar that will pass everything except a melt-down or an x-ray. Whether that will be cheaper then the underlying gold is an interesting question.

Apparently it all boils down to trust, where you buy from somebody you trust and then trust that somebody else will buy it from you when the apocalypse arrives.

Also it seems, based on cursory investigation, that it's not illegal to sell "counterfeit" gold. It's a commodity, not government-issued currency.