one is most places test for free. but the easy at home way to check is get your car jump started if its not starting on its own. after its running, pull the negative off the battery taking special car not to arc your wrench on anything. if its keeps running, alternator is good. if it dies, get it replaced. if the alternator is bad, it may have killed the battery, so get it tested and replaced if needed.
It's inside the alternator, if yours is bad replace the alternator or tear the alternator apart and replace the regulator.
bad alternator/voltage regualtor
Apparently the alternator is bad or the battery is shot, take the car to autozone and get a free charging system check.
How to Tell If Your Car Alternator Is Bad Check related link below…
old myth, bad for alternator, dont do this..
Bad coil? Bad rotor? Bad distributor cap?
you could pull off your positive battery cable if it stalls out then you alternator is the problem
A bad belt, a bad regulator in the alternator or bad brushes in the alternator.
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