A quick field test for a delco generator. Un hook the wires (and the belt if it is on the car) Hook jumpers to a decent battery, connect other end to the gen, negative to the case and positive to the positive gen post. It should turn on it's own like a motor. Not real strong like you could run anything off it but it should get spinning pretty good.
Another test is to just ground out the field wire. It should put out full at that point. I guess you would need a volt meter to see but you should be able to hear it pull with a good ear. Never rev up an engine with an alt/gen full fielded, you might let the smoke out of something.