Look for hallmarks. All authentic gold bars and coins have a hallmark stamp that indicates their purity. This stamp is a guarantee from a reputable manufacturer that the gold is 100% pure.

Costco has been selling one-ounce gold bars. The retailer limits them to two per member but they typically sell out "within a few hours," its CFO said. Rand Refinery gold bars cost $1,950 each ...

Sophisticated counterfeit manufacturers purchase real gold bars like these, hollow them out and fill them with cheaper tungsten. Pascal Lauener / Reuters. Sept. 19, 2012, 3:12 PM EDT.