It's really very simple. Ohm's law: V=IR. R is load, I is current, V is voltage. If you put on a large load (low R) and measure the voltage, that tells you the current being put out directly. The Autozone load test is fine, it creates a load. You can do very much the same thing by creating a load of your own, by turning all accessories on. Autozone's test can make a bigger load, so it is better, and it can vary it by computer and tell you the results in actual English words on the display, but you can get a pretty good idea by yourself at home whether it's working "well enough" or not with a simple voltmeter.
The only complication is the battery. That's why you need to make sure you measure voltage that is higher than battery voltage. That ensures that it is the alternator's ouput and not the battery's output that you are measuring. If voltage is higher than battery voltage, then the battery is consuming current and adding to the load rather than contributing current. The point at which voltage exceeds battery voltage is called the "break-even point" or "break-even rpm" because at that point the alternator's output is exactly matching the draw of all electrical components in the car. Below that point the battery is contributing electrons, above that point it is receiving electrons.
