The Stovebolt Site Forums The Shop Area The Electrical Bay Universal wire harness

I would hook the battery source to the terminal on the other side of the Amp gauge face discharge.
The 10GA to the terminal on the other side of the gauge face charge.
At some point you can turn on the headlights without starting motor. If the meter goes to discharge it's correct. If it goes to charge it's backwards. Ammeter will flow current either direction.
You can also use a flashlight battery, etc. Put the gauge in series with a 12V bulb.
Battery to one ammeter terminal, other ammeter terminal to bulb contact, bulb case (neg) to battery neg. Should show discharge if you have selected the correct ammeter terminals.