I have a Black & Decker electronic battery charger (10A max) I got cheap at Big Lots which has a "alternator test" function. I understand it just looks for a typical voltage signature across the battery leads that a functioning alternator would give (sinusoidal wave or such), so is more of a screening tool than a full test. To measure actual current output, I use an F.W. Bell clamp-on DC probe I got cheap on ebay. I would never pull an alternator before testing it in the car and maybe replace the Vreg & brushes first.