Check for battery voltage (with the ignition on) at the F/I pole at the connector on the back of your alternator. If the F/I pin isn´t getting battery voltage, the alternator isn´t going to work. There are at least two connectors between the alternator (F/I pin) and the ignition switch, maybe more (probably). Can´t remember witch pin it is, on the multi pin connector on the back, do remember it´s a yellow wire.
Another posibility is a coolant leak, don´t know why, but seems a anti-freeze bath will kill an alternator pretty quick. Took me two alternators to figure out what was going on with mine, pin hole leak in the radiator was spraying into the alternator, killed 2 pretty quick. Just talked with a buddy, who had a failed alternator after a anti freeze bath. I´ve had alternators full of water, even packed with mud that survived, anti freeze seems to be really hard on them.
Only a few ways to test an alternator, bench test (at the parts store), take it apart and test the windings, slip rings, brushes and diodes seperatly (a real pain) and swap out the regulator with a known good regulator (no way to test the regulator) or to try it in another early XJ (which should tell you if the external wiring is OK or not). There is a fusible link between the alternator and the junction block (starter relay)from there to the battery, also needs power to the F/I pin.
Is your trouble light coming on at the dash? Mine has a light that shows a picture of a garage, when the alternator craps out, in addition to a low reading on the volt meter.