Usually they used an amp meter on a generator and a voltmeter on an alternator (I read why once, don’t remember).
You’ll need an amp meter to tell you the generator output, but you can check the voltage with a simple VOM meter. Charge voltage between 6.7 or so and 7.5. Measure it with just the engine running and then add the lights and heater fan. Pick it up to fast idle or above. If it maintains the charge voltage you’re probably good. Remember, generators don’t charge much at idle, so if things look low pick the rpm’s up and look. Turning it up at idle will take you over voltage going down the road