To be honest, all I can think of is weighing them, but they could be filled with lead! So I guess I haven't thought it through... However, the Ebay seller feed

To be honest, all I can think of is weighing them, but they could be filled with lead! So I guess I haven't thought it through... However, the Ebay seller feedback count should be of some use. They could only sell a duff coin once (maybe twice) before being busted.