Battery light is on but tractor is not charing the battery. How do I know If it's the regulaor or stator or something else.

Hi,
If the battery light is on, it is not charging. The wire going to the regulator should have between 24 and 38 volts AC. The regulator should put out should have about 14.5 volts DC coming out of it(easiest to check at the battery itself at full throttle.)
Most likely the regulator. The stator will appear burned if you remove the flywheel to view it and it has failed. Do be cognizant of clean connectors to all components.
Peace,
Clarke