I'd say just take one of your normal planes out, and go up a bit high. Then make a couple of turns as you are now, with just aileron and elevator.

Then try a few turns by rolling, then feeding in rudder and elevator and turn that way, and notice the difference. Try it with differnet amounts of rudder, and you'll quickly see that it's not hard to pick up, and that you can actually easily see the difference most of the time. Btw, the flatter the turn, the more obvious the rudder input helps. If you're in the habit of making pylon turns, it won't matter as much

I agree that a rudder-only plane won't help nearly as much as must flying your existing planes with rudder.