Have you checked the voltage at the battery (meter to the positive and negative terminals)? If not do so and then move the red meter lead over to the battery post on the alternator and the black meter lead to the case. You should get the same voltage within a tenth or two. This will indicate if the case is grounded or not.
Best way is to do a voltage drop test to ensure that the alternator case is grounded and your connections are good.
Best way is to do a voltage drop test to ensure that the alternator case is grounded and your connections are good.