The Easiest way to tell when your car requires a replacement alternator, is to have someone jump start the car, then remove the leads, including the leads of your battery. If the car continues to run, then your alternator is doing what it should. However, if your car stops when you remove power from the leads, and your engine is running, then your alternator is not generating power, and requires replacement
Disclaimer: In certain situations this information could be false, as their are other electrical issues that could cause the same symptoms.
If you have successfully determined that the alternator is the problem with your electrical system, please remove it noting down the part number, and documenting a few photos of the unit. Come in with these details and we can sort you out a replacement part!