You need a battery/ starter / load tester, otherwise known as a "carbon pile." The "simple" idea s that you get the thing warmed up and "normalized" then turn everything off in the car that you can, then load the battery until the system voltage comes down to a specified value, 13.2 or 13.5 if not otherwise specified, and read the ammeter on the tester. Then figure in any loads in the car such as pumps, cooling fans, etc that you could not otherwise shut off