This product is aimed at precious metals manufacturers and refineries. At today's cost 100 gram 99.99 fine gold bar costs about 4400 USD, thus kilo bar is about 44000 dollars. There are no many objects competing with concentrated value of gold, platinum, palladium and silver.
If anybody else but manufacturers understand the value of their products and use various methods to protect the bars. There is always original refinery stamp and marks. There is also a serial number that is registered with manufacturer. All bars smaller than 1000 grams may come in special see through packaging. There is a assay kind of declaration proving that this certainly is a pure gold bar. Is it enough? Not by long shot!
Picture above depicts gold bar that would otherwise pass every surface chemical test with flying colors. External shell is made of 99.99 pure gold, however inside it cobalt bar that probably weights 90% of overall weight. "Manufacturer" of this piece is in China. Serial number and assay box and packaging have done a little to impede making this fake gold bar. It's all there. They actually count on new owner to never open such a nice retail packaging.
Only ultrasound or drill test would reveal an ugly truth. It is estimated that 50% of gold bars sold on Amazon and eBay are all fake Chinese bars!