Get a voltmeter. Measure your battery with the car off. It should be around 12 volts. With the car running and the blower, lights, etc. on, voltage at the battery should be around 13 to 15 volts. Any lower than 12, then the alternator is more than likely the culprit. Point being, voltage at the battery should be a little higher when the car is running, than with it off. This is one way to test the alternator. Also, check for bad grounds, battery connections, etc. Or better yet, go to Autozone and they'll check it for free. From my experience, if you get an alternator from Autozone, PepBoys, Advance Auto, or wherever, get the one with the lifetime warranty. If you get the one with the one year warranty, it'll stop working in one year and one day.