If you have a voltmeter to use, measure the battery voltage before you start the engine.
After you start the engine, you should see a rise in the voltage measured at the battery if the generator is working.Also, a generator will run like a motor if you apply the hot side of the battery to the field and arm. terminals at the same time..... of course, you want all wiring removed from the generator that is connected to the electrics of the tractor while your doing this.
If you run it as a motor, don't do it any longer than to see if it will run. I don't know what kinda tractor you are talking about, but.....most of the time, a generator either works or it doesn't and the rate at which it charges is set by the regulator. Does the tractor not have an amp gauge that you can watch to see if its charging? Ask the owner if you can have it checked before you pay him..... .....or...if the price is right, "take a chance"....50/50
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