Counterfits

It has been said so often that it probably qualifies as an urban legend: when Secret Service agents learn how to detect counterfeit banknotes, they don't spend months studying lots and lots of counterfeits to learn what counterfeits look like. They spend months studying real money, learning all the nuances that make real money look real. That way, if a counterfeit comes along that lacks those subtle characteristics, they instinctively know there's something wrong with it, even though they may not be able to point the finger at exactly what is wrong with it without careful analysis.

It is, perhaps, an oversimplification - you do need some experience with known categories of counterfeits to help ID them faster. But "experience" is what it all boils down to, experience in handling lots of genuine coins or notes as well as basic knowledge of counterfeiting techniques.

just as Sap said, if you look at enough of the real things you can spot differences that doesn't look right on the counterfeit coins. At first you may not know what doesn't look right, just that something doesn't quite look like it should. It is when you compare the counterfeit to a real coin is when you start to identify the areas that are not quite right. It is pretty much like everything else is within this hobby, there is no substitute or short cuts for experience