How do I check to see if my alternator is working OK?

From RPRWiki

First you need to test the voltage the battery receives while running (you can use the cigarette lighter plug as a test lead--I use a cig lighter plug, run the wires to the respective + / - on my multimeter).

I do this while driving and watch the voltages as I turn on and off my accessories, hit brakes, turn on lights, etc. The car should produce 13-14 volts operating, and 12v-13v with the engine off.

If the voltage while running is 13 to 14v then check at the battery while it is running, it should also be 13v to 14v. If it is 11v to 12v or lower, your alternator wire or fuse link behind the battery is gone/going.

If your alternator is not putting out 13v to 14v then it could be one of two things: 1. Bad alternator. 2. Bad regulator PLUG pigtail/wire (the plug in the alternator).

Either way you should remove it and take it to the nearest parts store to have it bench tested. If they say the alternator is good, then it is your plug. If they say it is a bad alternator. Then check the ohms (continuity/resistance) of the wires of the alternator plug and bolt-on wire (should be 0 ohms to 0.03 ohms---a.k.a. no resistance). If the resistances are within tolerances, then check your battery. If the battery does not check out, it will cause the car to run solely on the alternator, and cause the same symptoms described.

In other words, you are using up the alternator power and the battery isn't enough to catch it, so the output is solely dependent on the rpm's ---more speed, more charge, less speed, less charge. My car behaves this way. But I know that it is from my 400W per channel amplifier and my 10-inch subwoofers, 6x9's, 8" speakers, and neons. While on --with wipers, fan, and headlights the voltage may be around 11v and dropping, except during high RPM. When I turn them off, the voltage goes back to 13v.