There are a couple of ways you can do this. One is by using a multi-meter set to DC Volts. If the meter shows less than 13 Volts it is not charging(this is done with the engine running). Another indication is if the icon on the dash that looks like a battery illuminates then the alternator is not charging. Typically the alternator will produce 13.5 to 14.7 volts. If it does not you will need to replace the alternator. Make sure to check this after installation.