HeadlessRoland
Shooter
Winner! In fact many gold bars are fake nowadays. I think they use something other than lead but always cut into a bar before you weigh it if buying. Or use another method to dermine it has the proper weight for a given volume.
Fisch makes a great anti-counterfeit series of devices that check silver/gold coins against a series of four checks. Then again, there have been counterfeit Fisch devices pop on the market too. Always check any bullion purchased with your own device, never let a stranger weigh or check any coin without verifying their results independently. Biggest risk of fakes - especially for coins - is the use of tungsten, whose density almost matches gold's (19.25g/cubic centimeter versus 19.32g/cc for gold) and makes it very easy to plate a brick or shape of tungsten with a small plating of gold, and if done correctly, the weight can be made to almost perfectly replicate for a gold bar of the same size.
There is already a wide market for just this sort of "gold substitution," in fact:
http://www.zerohedge.com/news/tungsten-filled-10-oz-gold-bar-found-middle-manhattans-jewelry-district
Gold-plated Tungsten-Tungsten Alloy