These tests shown here are for a Cub 6 volt Pos. ground generating system with regulator. They may work with other systems but care needs to be taken. Even polarizing some systems is different and if done improperly the regulator can be shorted out.
I’ve set up these tests to basically correspond with the flow chart that is often referred to and posted on this site. ( Find it here. download/file.php?id=9047&mode=view )This may or may not be the best way to test but I figure it is some thing we are all familiar with. Also doing it this way you don’t have to have a meter, but it would be helpful if you suspect or you determine that the gauge is the problem.
One thing I’ve noticed in my experience is that it isn’t uncommon for an AMP gauge to work in one direction and not the other.
I may be editing this and the schematic tests. I had thought of something in the middle of the night that could be of help, but now I don't remember what it was.