The gauge is run in series with the alt lead to the batery or somewhere in that line usually and you can see the negative draw when cranking usually around 30 amps then once started and the alt kicks in you can see that its charging the bat at usually just under 30 amps until the bat is recharged then the gauge usually hangs just in the positive zone, apm or 2, depending on how many current draw things you have going. Turn on the headlights and you can see the gauge flick just for a second until the alt corrects for it. These gauges are a little dangerous as you have to run the hot wires all the way into and out of the car or wherever the gauge is and if one of those wires should get chaffed and touch metal its a dead short so there needs to be a big fuse or link somewhere in the line.
HTH
