I have no idea if this works, but it got good reviews on Amazon where it's $15 shipped. Here's the blurb:
Now it's a simple task to check the condition of your car's battery and the state of it's charging system with the Peak 12 volt Battery/Alternator Tester! This tester will check your car's battery before and after charging. It also checks the alternator, engine start and battery charge level. It plugs directly into the 12 volt outlet of the vehicle.
•Plugs directly into the 12 volt outlet of the vehicle
•Checks status of battery like full/partial/low charge
•Indicates status of battery
•Checks status of alternator: Good/Faulty/Over-charging
•Safe, easy, effective