From Felix Salmon, back in March, here:
If you can weigh the bar accurately, then you can test for purity by essentially dropping it in a bucket of water and seeing how much the water level rises: a gold-covered tungsten bar will displace more water than a pure gold bar. Alternatively, for $3,000 or so you can buy a micro ohm meter, which is easily sensitive enough to tell the difference in conductivity between a pure gold bar and one which is largely tungsten.
