Is there a little test one could do to see how substantial the effect fo propeller momentum is on the a model?

I can think of one.: Run the engine at idle, pick up speed in a dive, now quickly yaw the plane and observe the resulting pitch that occurs as a side effect. (It may not pitch at all.)

Next do the same thing but this time flying straight at level, or on an up line, with the engine running at a higher speed. Yaw the plane the same way at the same speed and observe the result. If it pitchs more than when the engine was running slowly, then there is a noticable gyroscopic force.

Unless... of course, the prop blast has some effect that cause the plane to pitch. I'm not sure if that can be ruled out.