Find a dark spot at night. Turn the headlights on and the heater fan. With the engine running, watch ahead as you speed up the engine. Then let the engine idle as you turn the fan off, then shut the engine off. All this time observing the brightness of the headlights.
All this with the truck sitting still and you in the drivers seat of course. This is a quick, easy and safe way to give you a good idea. Of course the best thing would be a working factory ammeter.
