Can you check the output of an alternator by using a voltmeter with one lead on the output wire and the other going to a ground?

yes or just put the voltmeter right on the battery. either way if you have anything less than 13.5 volts there is a problem. most of the time I find mine to be charging about 14.2 when hot and 14.6 when cold.

Next, run a jumper wire from the VR screw labelled "GROUND HERE TO TEST" to a ground (like the alt case) while monitoring voltage from the case to the output post. With the engine at high rev, it should jump suddenly toward 18V. If it does, disconnect the wire immediately. For a more-horough diagnosis, call a few local parts stores to find one that will test it free in the parking lot.