About the only checks that can be done with the alternator out of the engine are, a) a continuity check to make sure the windings are sound, and b) an earth leakage check to make sure the individual coils are not leaking to earth. Even having done these checks, there may still be problems with the stator. On a unit I was intending to use on a bike that has been converted to 12 volt, the unit functioned correctly to start with, but then gradually faded until no charge was being generated. Having been washed out and blown out with compressed air, the same would happen again - I came to the conclusion that the unit had come from a badly worn engine and had become impregnated with metal laden sludge which, under the effect of the magnetic field, would gradually produce a track to earth. I considered using a modern replacement but the new units are slightly fatter than the original Miller unit, and there wasn't enough room in the timing chest - so the only solution was to rewind the Miller coils (which I eventually encapsulated in resin to prevent any re occurrence of the original problem). The new coils have the same gauge wire and the same number of turns as the originals, but the rewound unit gives a good 8 amps charge at 12 volts - 96 watts (not bad for a unit that was originally rated at 60 watts!)
Eddie.