This is easy - you can determine the functionality of the alternator in less than 5-minutes. With the engine off, using a volt meter switched to measure DC current, place the red lead against the positive battery terminal/post and the black lead against the negative battery terminal/post and test the voltage. A good battery should measure around 12-volts (a good battery has a range of slightly lower to slightly higher than 12-volts). If you get within this range you have a properly charged battery and can then move to the next step and test the alternator.
Turn the engine on and again place the red lead against the positive battery terminal/post and the black lead against the negative battery terminal/post and test the voltage. The alternator is designed to generate enough voltage to replace the 12-volts of power being used by the vehicle's electrical system and charge the battery at the same time. Therefore, a properly working alternator will push about 14-volts of electricity into the system (a properly functioning alternator will generate a range of slightly lower to slightly higher than 14-volts).
If the system is generating around 14-volts with the engine running, then you can reasonably conclude that the alternator is working properly.
If you you are having battery charging problems (will not hold a charge) and the system is pushing around 14-volts with the engine running, you can then reasonably conclude that the battery needs replacing. If the system is producing less than the normal range of about 14-volts, you may need to replace the alternator. Keep in mind that a good battery may go bad if it is repeatedly used in a system in which the alternator is not working properly. If the systems tests within the normal range of 14-volts while the engine is running, replace only the alternator first and see if the battery then holds a charge. If it doesn't, then you will need to replace the battery also.
Perhaps thids might help... http://wiki.answers.com/Q/How_do_you_tell_if_the_battery_or_the_alternator_is_bad
There are generally two signs that the alternator is going bad. First is the noise. A new noise will begin to be heard. Something like a whining when the car is accelerating. Second is the smell. You may smell burning from the engine department.
When your battery does not get charge.
If bearings go bad they will "squeal"
There are a few reasons why your alternator might keep going bad such as rusty latches under the car. You may also have leaks, dents, or parts that need to be replaced on the vehicle that are effecting your alternator.
Take the belt off and spin the alternator by hand. If you hear a grinding noise, like sand rubbing on metal, its probably going bad. Also try to wiggle the pulley up/down/left/right. If it wiggles, the bearings are going bad. Of coarse you can always wait until they get so bad they shatter and the alternator wont spin at all, like i did. :(
How to Tell If Your Car Alternator Is Bad Check related link below…
Normally this is the bearings going bad.
If the alternator fuse is bad, the alternator is not going to work properly to charge the battery. An alternator runs off the serpentine belt, with graphite brushes inside that conduct electricity to charge the battery.
There are several signs that an alternator is going bad. A few signs are dim lights, smell, warning lights, and a dead battery.
Alternator is going out? battery is going bad?
You can take it down to a shop and get it tested, but if you can not afford that, you can take the negative battery cable off of the battery while the vehicle is running. If the vehicle dies, you may have a bad alternator.
A bad alternator will make whats called a whirring noise. It could also make a low growl. This is just the bearings going bad. If your alternator is not supplying any power, it has nothing to do with noise typically.
A bad belt, a bad regulator in the alternator or bad brushes in the alternator.
The diode in the alternator is probley going bad, Check charging system. And replace alternator.
One way to tell if your alternator is going bad is to start it up, preferably at night, and turn on the lights. Then turn on the the heater fan. If your lights get dimmer when you turn on the fan the alternator could be going bad. A common problem with an alternator that is insufficiently charging is having to jump start your car even after you have run it beforehand. The best way to test it is with a volt meter. You should read 13 to 14 volts across the battery while the engine is running, if you read less your alternator is insufficiently charging. Another way to test it is to take it to a parts store for a free test.
There could be many reasons your battery is going bad or your belt is off or you have a bad alternator.
Usually it means your alternator is bad or going bad.
i have an 87 topaz where the alternator goes bad also. have replaced it twice. it is just a common problem in the topaz's
you could pull off your positive battery cable if it stalls out then you alternator is the problem
The bearings on the alternator could be going bad. Remove the belt from the alternator then spin the alternator by hand to listen for grinding.
The alternator is going bad, may need to be replaced soon
Typically, a bad or "going bad" alternator.