How do you test an alternator?

Start the engine, while the engine is running, remove either battery cable. It should stay running, if not, possible bad charging system. Or take it to auto zone. They check them for free

NEVER EVER disconnect a battery cable while the engine is running unless you like your computers fried.

Here's a link to the proper way to test:

http://free-auto-repair-advice.blogspot.com/2007/09/avoid-bogus-alternator-test.html

Saying again, never ever disconnect a battery or alternator with the engine running. The resultant voltage spikes can fry electronics, the alternator diodes can be overloaded and fried...bad things can happen. The don't ALWAYS happen, but why risk $1,000's of dollars in damage when there are correct ways to test your alternator.

One simple way is at night star your car, turn off all electrical accessories, turn on your high beam head lights and point your car at a wall where you can see the reflected light well. Notice how bright they are. Turn off the car. If the lights get brighter, odds are very good your alternator is bad/weak.