Measure the output of the alternator at the large wire on the back with a multi-meter and if it's working you should get 14.5 volts or a little less at idle.
And's advice is great and it's easy to do.
If you want to keep your battery gauge, and it works you can tell if the alternator is charging by looking at the battery gauge, and with the engine off if you tun on the headlights, the gauge will swing left toward D (discharge), and when you start the engine it will come back to center and steady at the first mark right of center. If it doesn't work. Then study this thread and it will help you understand how the battery gauge (also called an ammeter) works, and is wired in the harness.
http://67-72chevytrucks.com/vboard/s...d.php?t=601025