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To properly test alternator output, first lightly discharge the battery with the engine off and with your headlights on for about two minutes. Start it up and immediately apply all the electrical load you can i.e. high beams, high fan blower speed with an engine speed of 2000 rpm. Look for 13.2 - 13.4 volts and initial amperage near the rated output of the alternator. The amperage should slowly come down as it recharges the battery. If the amperage drops quickly and stabilizes to the remaining load, it could be a sign of a weak battery. If you have a battery with caps, hydrometer testing will give you a decent idea of the batteries condition. Surface charge it first and leave it rest 20 minutes before testing.
You can decrease load for driving under low RPM conditions by increasing the diameter of the alternator pulley but you will need to make up the charge either by higher RPM driving which would be a wash from the economy standpoint, or by externally charging later (solar would be the best IMHO).
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