Well, you and I know of the specific gravity (19.6) of gold and how dense it is.
Realistically speaking, though, how many folks have ever held five pounds of gold in their hands? or even five ounces?
If there is any kind of breakdown, people would go into survival mode very quickly. And that means desperation. And throwing values and morals to the side.
The amount of swindlers out there depends on one thing - the amount of suckers! And we know there is no shortage of those!!!
If I can't convince you of how easy it is to tell the difference between a real Krugerrand or Eagle etc and a fake, then I don't know what to say. One weighs double the other. Plus, I've never seen or heard of counterfeit Krugerrands etc, have you? Perhaps you can point me to examples. A minting machine which could exactly duplicate a known gold coin would cost over six figures, minimum. Cast coins would be spotted at a glance, a cast fake can't come near the detail of a struck coin. Then, after you have spent 100s of thousands on your minting machine to make fake gold coins, your lead core fakes would still weigh only half of the known weight of real gold. A simple postage scale would be all the technology required to tell a fake from a real coin. In fact, you can tell in your hand the difference in weight, gold is almost double the weight of lead! Really, this is not a problem. This easy discrimination is one of the reasons gold has been preferred for coinage over thousands of years, it can't be faked. You just can't pack an ounce of lead into the space occupied by a one ounce gold coin, try as you might. It's pretty basic.