The most accurate test is done with a high-rate ammeter, and enough load on the battery to put the alternator into full output, but that type of test equipment is usually only found at a professional shop. You can do a rough test by turning on all the accessories you can, like high fan on the heater, high beam headlights, cigarette lighter, etc., and look for a low output voltage, but it's hard to get enough load on a system for that to be a valid test. If in doubt, have a pro do a "full field" test on the alternator, and see if it can produce its rated current flow.
Jerry
