You can take the alternator to any of the AAP, AZ, O'Reilly's and they bench test batteries, starters and alternators for free. You can also take the whole car there and they can do a system test, battery, starter, and alternator test on the car. It takes 2 minutes and all of these services are free. They also do code checks for free too. I do it myself and give them my driver's license and they let me use their code reader.