Start by measuring the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If that is okay but it's still not producing enough current, suspect a defective internal diode. You will find that with a professional load-test. The most current you'll get will be exactly one third of its design rating of typically 55 - 60 amps, and "ripple" will be very high.
You can also bypass the voltage regulator to see if that is the cause of low or no charging. Jump the "A" and "F" terminals together in the regulator's connector. If the generator charges wide-open, replace the regulator. Do that only long enough to get the results, and do not raise engine speed above idle.