At many auto parts stores they will test - often for free - a vehicle's electrical system by attaching a device to the positive and negative terminals of the battery. That's all they do. Then you turn on your vehicle, and, like magic, they can tell you if your battery, starter, or alternator is faulty. Not only that, they can provide very specific information regarding any faults.
Is the information produced by these systems meaningful?
What if more than one component is actually faulty?