If the voltage metere is moving up and down there a good chance that its the alt. You can also place a volt meter on the battery while the motor is running (with every thing off), look and the voltage, then turn your A/C on high, turn your lights, wipers, radio ON, if the voltage goes below 12.5, there a good chance that it is bad.
How to Tell If Your Car Alternator Is Bad Check related link below…
A bad belt, a bad regulator in the alternator or bad brushes in the alternator.
you could pull off your positive battery cable if it stalls out then you alternator is the problem
Start the car, unplug the battery the alternater should power the car. If it dies the alternator is bad. You can also get it checked at any parts store.
The quick easy method for determining a bad alternator or at least to find out if it is charging, while the car IS running remove the negative battery cable from the battery, if the car stalls then you need to have the alternator service inspected.
Get engine running. While the engine is running disconect the POSITIVE termninal from the battery. If the engine dies, the Alterntor is bad.
Perhaps thids might help... http://wiki.answers.com/Q/How_do_you_tell_if_the_battery_or_the_alternator_is_bad
bad alternator, bad voltage regulator, bad battery, bad connections.
The best way is to drive it to your nearest auto parts store and have it tested. You can test it somewhat yourself. Start the engine and use a volt meter to test the voltage that is being supplied by the alternator for the battery. If it is less than 13.5 volts the alternator is bad.
alternator is bad
Start the vehicle then disconnect the battery. If the motor dies, you probably have a bad alternator.
a bad ground wire can ruin an alternator by not completing the circuit