how do I know if my alternater is going in my car

you can test the battery in any part store for free also if the alternator is not charging the red light with battery symbol would be on in the dash .. also if the battery is good and after driving a while won;t start that mean there is no charge

Usually a check battery or engine light will come on if the alternator is bad, I would start with a new battery

You'll need to use special equipment that measures the output current of the alternator, and have the battery and battery cables checked as well. Most shops can do this for a relatively small fee ($25 - $75)