I would not "bet" either way. Check that the "insulated" one is not actually shorted/ grounded (missing/ broken hardware) and check that you have field continuity to ground. Make careful checks as the resistance of the field is fairly low. In other words do not mistake the low resistance of the field for a short and vice versa.
There has been lots of stories about aftermarket/ rebuilt alternators assembled incorrectly with incorrect or missing hardware. Sometimes the "grounded" one has the insulators installed and is not really grounded in that case.........and should be
