Im going to store all of my gold dissolved in a mixture of hydrochloric and sulfuric acid (Aqua Regia) the thieves would have to know how to turn Chloroauric acid back into solid gold powder. You could carry a vat of aqua regia around with you when doing business and throw any chunks of gold into it, this not only confirms the purity of the gold, but also is a convenient way to purify and measure exactly what you are getting.