often the materials coins are made of are chosen in order to make counterfeiting them counter-productive. in other words, to counterfeit a $1.00 US coin would require $2.00 worth of the same materials (obviously the government buys the materials at a huge discount). so counterfeit coins are made from cheap materials, and so have different physical and magnetic characteristics. measurable characteristics would be magnetic, size, mass, optical reflectivity and absorption (not reliable because of oxidation and wear). the mix of ferromagnetic and paramagnetic characteristics would probably be the best method), and measuring the density (by determining the volume and mass, and then calculating the density). you should see if you can find out how coin counters work. most large businesses use them. there are a very small number of tests that can be done in the few hundred milliseconds that it takes a coin to pass through a coin counter. if you're trying to come up with a better way to detect counterfeits, more power to you, but most of the tests already done have been around for a long time, and work quite well