A while back I found a neat coin tester on the Internet. I can't find it now, but it worked by testing shape/size and weight. It was a simple 'card' that had a pivot point on it and a well in which you put the coin. The size of the well fit the coin exactly and the card would balance if the weight was correct. This sort of thing would fit in a pocket and might be able to be made with a second well for a different size/weight.
We could/should also have one that does size/shape plus weight plus some electrical testing. Apparently conductivity changes when composition does. This sort of tester could be useful in a retail situation as it could be built to test multiple "coins".
It has also been mentioned that a register could be built that has the testing stuff built in. It can simply weigh the tray to give an accounting of the register's balance (and also tell if the cashier is stealing from it!).