Ken,
You can also check charge rate by checking the voltage. Most alternators on newer gear regulate voltage between 14- 15 VDC. If you have a generator, it will operate roughly the same, but usually has to run at a higher speed to produce juice. The battery should be at or above about 12 to 12.25 volts if its in good shape (a new, fully charged battery will run 13+ volts). Once you start the tractor, check it over several minutes. The voltage should approach 14 volts or so. Be sure to give it a little throttle so the generator is running at a high enough rpm to put out well. If it is below 13 volts, I doubt the gen is working, or the regulator may be bad. If its above 15, the regulator is out of wack, and you will end up overcharging and cooking the battery. Any voltage above 13 to 13.5 and below 15 VDC is in the ballpark.
Good luck.
DaveL