First hook a DC voltmeter up to the battery. With the engine running you should see 13 to 15 volts at the battery. If you see less than this, than I would be checking your alternator and regulator. You can also check the stator in teh alternator by measuring AC volts at the battery. If you read more than half an AC volt, than the stator is probably bad.
You can check for battery drains by doing the following:
Make sure the doors are closed and the ignition is in the of position. Disconnect the negative cable and attach a test light between the negative post and negative cable. If the test light comes on then you have something draining your battery. If you have an aftermarket radio with memory then this will draw power even when everthing is turned off and can cause the test light to turn on. The power draw from the radio should only be about 40 milli amps or less and this is not enough to drain a battery overnight. I recommend disconnecting your aftermarket radio (easiest way would be to pull the radio fuses) and testing again. If the light still comes on, you can narrow down the source of the drain by pulling one fuse at a time till the test light doesn't light up.
