Run a battery hot wire to the red post, and a battery ground wire to an uninsulated post or a good ground on the block. With the engine running, run a hot jumper wire to the white "FLD" post while monitoring battery voltage with a meter. If voltage shoots up, you've got a keeper. Only do this long enough to get results, as leaving it on will cook your battery.
If the voltage does not go up, shut off the engine and touch the hot jumper wire to the white terminal. You should see a spark when you do. If you don't, it has bad brushes.
Keep in mind that this style of alternator requires an external regulator.
