It depends on if its Pos or Neg ground. If an ammeter registers bass ackwards, all you do is reverse the leads. If on a tractor etc that has a correct wired ammeter if you turn lights or ign etc on when she's not running, the ammeter should swing over to the - discharge side but then if running and the genny works it will swing to the + charge side and if it does opposite of that simply reverse the leads. The ammeter is just an in line series passive device to register current flowing INTO/charging the battery or out of discharging.
Regardless and to standardize things REGARDLESS OF POLARITY I tell people theres ONLY ONE wire on the SUPPLY SIDE of the ammeter and it wires to the hot UNGROUNDED battery post. On the other LOAD SIDE theres often TWO WIRES, one to loads like lights and ignition, the other to the BAT terminal on a Voltage Regulator.
To work BOTH the gennys case frame and the VR case frame must be grounded i.e. connected to the frame grounded battery post or frame ground connection.
On some trucks/cars/tractor wiring diagrams I've seen on NEG ground systems the ammeters + wires to battery + and loads and gennys output (BAT on VR) wire to the - of ammeter.
If its an A system my Troubleshooting Procedure in Para 5 will tell if non charging is a genny versus VR or other problem.