Alternator question

Harbor Freight has a little "plug into cigarette lighter" indicator w/ LED's for $6. It will show if the off (battery) and running (alternator) voltages are good. I just bought 2 more on sale, and keep in all my cars, especially the new ones that have no such dash meters. I also have a Black & Decker battery charger that has an "alternator check" function. Autozone has similar to give free checks in the parking lot. They just look for a sinuosidal pattern which indicates the alternator might be alive. It isn't is valid as the ammeter in our A's. If I really want to know, I have a clamp-on DC probe to measure the true output current.