The standard way used in public telephones is to have the coin roll down the shoot between two series connected coils,which are part of a colpitts oscillator.By measuring the change in oscillation frequency and the damping of the oscillations,as the coin rolls down the shoot you can detrmine the conductivity of the coin.At high enough frequency,approx 1Mhz the skin effect allows you to measure the thickness of the coin.
I worked on the coin validator for all BT public telephones where people where wrapping coins in metal foil to make them look like higher value coins and using the above scheme we were able to get a system which rejected only about 2% of valid coins,and rejected 99.5% of all false coins.