RT,

I would simply drive the car to an Auto Zone (or equivelent store) and have the guy at the desk come out to your car (parked relatively close to the front door) and have them do an alternator output test (includes various load simulations). This will be at no charge and will definately tell you if your alternator is bad.

Your in car gauge is a pretty good indicator if an alternator is failing. I ran into this same situation on a 1991 Buick Lesabre. After determining it was the alternator, purchased and replaced it. However, the in dash gauge still had an infavorable reading. I didn't like the fact that the new one read only 12 volts at idle. In reality, alternators go through manufacturing - like all parts do - and have to pass quality inspections. These inspections usually have a tollerance range. This new alternator has met the absolute bottom of that range and is unacceptable. A new alternator should output above 13 volts at idle on a properly operating charging system. I brought the alternator back to the parts store and, sure enough, they tested it; agreed with me and gave me yet another alternator. After installing this one, it read 13.5 volts at idle (no load). Vola!