[Tommy]

Doug,
I have two suggestions. First is to look for any identifying part numbers that you could use to research the capacity of your alternator. They would be the same numbers you would use if you wanted to go to the parts store to replace it.

Second, think of the battery as a reservoir of electrical power that fills in for the alternator when it is not making enough power (e.g., when starting the engine), and takes back power from the alternator to refill the reservoir when the alternator is making more power than the other electrical systems need. That means an underproducing alternator would leave the battery power lower after running the engine than before. As Dwight said, you can read that effect with a voltmeter. ... Here's what I would do. First, fully charge the battery with a charger. Let is sit for a while to reach its steady state and then check the voltage (e.g., 12.4 volts). Then start the car, let it warm up a bit and then check the voltage at the battery with only essential electrical systems on. Check it at idle and at about 2,000 RPM. It should be above 13 volts. Now drive the car for 30 minutes or more without stopping or restarting the engine. When you do stop, turn off everything and check the battery voltage again. If it is less than the the voltage of the fully charged battery (e.g., 12.4), then the electrical system is not sufficiently replenishing the power reservoir in your battery. This could be due to a bad battery, failing alternator, undersized alternator, or bad voltage regulator. If I had these symptoms I'd first have the battery checked and, if it passed, I'd have the alternator output checked. Most chain parts stores can do this.