well if you connect the field to the battery. the generator will give full (unregulated) output. the voltage will rise with the RPM if the field coil is good and the stator is good. it could go as high as 40-50 volts so be careful doing this test
the field windings measure in the 4-8 ohms range, whatever color the wires are. I think one is white & the other appears to be green. hook one wire to battery plus and the other to ground.
measure the VOLTAGE as you run the engine. if the output voltage rises with RPM & goes high, then the field and (probably) the stator are good, if not then one of them is bad.... probably the stator if the field measured something like 8 ohms & is not shorted to ground in it's own case/frame. the voltage can go as high as 40 or even more so don't dally at high outputs or the lights will cook
do this test & if it fails it is either the stator or the field windings. if it passes then the problem is the rectifier, the regulator, or the wires to those systems. there are other tests to identify the problem from here.