Got a voltmeter? I'm surprised nobody suggested actually measuring your charging voltage.

Engine running, should be above 13 volts.
Engine running, turn on everything you can find. Lights, wipers, blowers, amplifiers, windows, whatever. It should not drop much.

If it can't hold 13 volts when the engine is running, your alternator isn't working. First check all the wiring connections at the battery, alternator & ALL the grounds.