Put a DC voltmeter across the battery with the engine running and look for about 13.5 to 15 volts. If it is more like 12 volts, then the alternator is indeed not charging.

If alternator not charging, check connection at alternator, and remove and inspect the voltage regulator assembly from the alternator. You can do this without removing the alternator. There are two screws that hold in the voltage regulator to the back of the alternator. Once removed, see if the brushes are worn out. This is the most likely cause. If so, click on FastLane above, then online catalog and you can get a voltage regulator for $14.

Good luck,