Old coins can be worth a lot of money. But at Ye Olde Curiosities Shoppe in South Orange, proprietor Arnold Saslow has noticed a disturbing trend: customers bringing in fake coins worth nothing at all.

"People come in and say that they were approached either at the bus stop or train station, sometimes even as they were stopped at a traffic light," says Saslow. "Somebody would give them a story about how they came on these rare American silver coins."

According to Saslow, they look like the real thing, but they're made of steel, imported from China. He warns, "Don't buy coins from people you don't know on the street, because the chances are 100 percent you're going to be offered fake coins," says Saslow.

Counterfeit coins may also be for sale on auction sites like eBay. One red flag to watch out for are offers that are too good to be true. According to Saslow, if the coin is dated hundreds of years ago, around 1800, it's almost definitely a fake. 

You can also test so-called silver dollars by simply using a magnet. If it sticks, it's steel.

Unfortunately, you may have to buy the coin to test it. So a good rule of thumb is to never buy coins from someone you don't know and trust.