Your first question, how do you verify the the rotor RPM is constant answers your second question how do you do it. You verify it as simple as it sounds if you want exact results get an optical tach and someone to assist you. A govenor will certainly assure constant RPM just as you said involving tweaking and the verification being an optical tach or someone who swears they hear your target RPM coming from your engine.

Think about driving a stick shift car, if you can't hear or feel the engine then you look at the tach. If you can't hear the engine and you can't see the tach then keep guessing. How much does it matter? Well when you stall your blades it matters just as much as when you lost a race by .01 seconds in the long run... If you're going to troubleshoot a couple degrees of pitch or a couple RPMs you better start investing in the non "cheaply" methods of detection....