What you whould most likely use is the conductivity / permeability of the metals. Modern metal detectors are very good at seperating out metals this way.
Each alloy will have a "natural frequency" with which it reacts to changing magnetic fields.
If you place one of the balls near an electromagnet, then pass current through the electromagnet such that the resulting magnetic field passes through ball, and then suddenly turn off the current to the electromagnet, the magnetic feild through the ball would not disappear at once. Instead, the magnetic property of the ball will have an effect as would an electrical current which would start flowing in the ball when the magnetic field attempted to callapse.
The magnet field in the vicinity of the ball would show an exponent decay. From the rate of this decay, you can do a great job of distinguishing one metal from another.
Of course, if you want to know more about the metals, there are machines (even hand held ones!) that use radiation to excite the electrons in the metal, causing them to give off x-rays. A detector determines the wavelength of each incoming x-ray by the amount of electrical charge given off. After enough x-rays have been measured, the machine goes through some confusing math and tells you elements are in the metal (at the surface).
Of course, if you want to be boring, you can check the metal for magnetic influence, resistance to acid, hardness...