start engine and measure voltage, either across battery terminals, or
between a good engine ground and the alternator output. should be about
13.5 to 14.5 volts - if so, alternator output is ok. may have to rev up a
little above idle rpm to achieve this. final test is with lights, high
beam, blowers, and so on, to produce maximum load.

notes: it's not a good idea to run engine with battery disconnected on
modern cars, even an '84, because it acts as a ballast to protect
electrical system. even if engine keeps running, this does not prove that
the alternator output is adequate.