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Set you meter for the DC scale that is closest to, but OVER 12 volts. With the car off, check the voltage on the battery. This will tell you what voltage you have without any drain or charge occurring. Take your meter off and start the car. Now check the voltage at the battery again. It should be a slightly higher value - say 14V or so. If this is not the case, there is a problem
Background:
Power is a function of Voltage (volts) and current (amps). In the real world, the 12V system in your car will vary anywhere between 11.5V ~ 14V or so. Sensitive electronics will drop the power down to a lower voltage level and provide additional circuits to filter out any unwanted power fluctuations. Your main battery is a Power reservoir that is used to start your car, and power all of the other circuits within it. Like any other reservoir, it needs to be continually refilled in order to be available when needed next time.
The job of "refilling" your battery with power is done with the device we call an Alternator. An alternator takes some of the rotational power of the engine, and converts it to electrical power. The device in your car actually generates alternating current (AC), and is converted to direct current (DC). On a graph, the AC looks like a sine wave with vehicle ground (negative terminal) being in the neutral or middle. With the miracle of semiconductors, this current is internally passed through a full wave bridge rectifier, which takes the bottom half of the wave and flips it to the top of the ground. (In the mechanical world, this is like a pump that uses valves to pump a medium on both the push and pull stroke.) The signal now looks like a continuous series of arches that go from 0V to ~16V at a relatively high current rate. This signal is put through some circuitry that will smooth it out to a good facsimile of DC voltage, and regulate it to about 14V - again, with high amperage. This is presented to the terminals of the output of the Alternator. This power is channeled back into your battery via charging circuitry.
Bottom line is this: The location where the power enters the battery is the same place that it leaves the battery, at the terminals. This is where the rubber meets the road so to speak, and where you can measure the end-game of your charging system.
Good luck.
Peter Haakonson
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