First thing, I agree with the others POLARIZE THE GENERATOR before you do anything else as there's a good chance at the shop it got polarized for Negative ground !!!!!!!!
"If an amp meter shows a discharge when you turn on the lights does that mean there is nothing wrong with the meter."
It don't prove the meter is absolutely perfect and will indicate a positive charge HOWEVER it tells me its probably wired at the correct polarity.
"Doesn't show charge on my tractor so is it the meter or my wiring."
You can tell if its still charging regardless if the ammeter works or not. Put a volt meter on the battery tractor not running and if its charged it should read around 6.3 volts. Then start her up and at a fairly fast RPM if the charging system is working battery voltage should rise to at least 6.5 to near 7 or so volts subject to battery state and RPM and charging system. If it never raises voltage SHES NOT CHARGING
Finally to tell if non charging is a generator or a Voltage Regulator or cutout or control problem run through my Non Charging Troubleshooting Procedure linked below. It explains in Para 5 how to by pass the VR or manual Low High light switch LHDB charge control by dead grounding the gennys FLD post to see if its a gen or wiring or VR or control problem.
"I only have a wire from the regulator to one side the amp meter and a wire from the other side to the starter." Typically the VR or Cutout Relays BAT terminal wires to the Load (NOT battery/starter) side of the ammeter while its other Supply side wires to battery often where the big cable attaches to the starter switch/solenoid input. That would allow her to charge if alls well.
BOTH the Genny and VR or Cutout Relay need good grounds, check for that !!!!!!!!!!!!!!!!
Run my Troubleshooting Procedure should answer your questions