Use a multimeter to directly measure the positive and negative voltages of the generator to determine whether the generator is producing electricity normally.
We know that when charging the battery, the generator output voltage is higher than the battery voltage. If the rated voltage of the battery is 6 or 12 V, the output voltage of the generator will generally be 7.2 V and 14.4 V. Therefore, as long as the measured voltage of the generator is about 1.2 times the battery voltage, the generator can be judged good.