Ping Test
Got my coin test tool in the mail today and verified that it works. I used Pingcoin when I was buying gold in Seattle and verified this latest version today, too.
The tool hammer strikes the coin and the phone app listens for the right resonant frequencies to verify the coin is authentic.
Gold And Silver Coin Pinger - Acoustic Ping Tester With Built In Coin Striker | eBay
Its compact and lightweight design makes it easy to carry around and use whenever you need it! You should hear a long ring, if the coin is genuine. If the coin is fake then it will not ring at the expected frequency.
Pingcoin - Apps on Google Play
Digital Ping Test for Gold & Silver Coins