The only way, is to cut through the guess and by golly and use a methodical approach. RTFM! There aren't any shortcuts, at least until you reach a point where it's all memorized. You have to have a voltmeter, no way around it, and if measuring anything other than the charge voltage at the battery posts it really needs to be a needle type analog meter. A digital display is going to hork out on some of the tests when trying to test a mechanical voltage regulator. With the generator on the bench (or on the truck) there's some tests you can make, for example spinning it up with a drill and measuring output, on a Ford type for example by tying the ARM and ... is it the FLD or GND with a jumper wire together? See, I can't remember. On type A generator's it's different. Ya gotta have a manual or you'll roast something. Can also see if the generator will motor if 12 volts is applied. These aren't definitive tests but if the generator motors OK it should work.