ORIGINAL: kriegsmacht
Easy, just epoxy a prop to a dowel rod. Make a wooden frame consisting of a base and 2 upright supports for the rod. Holes are drilled in the frame to accept the rod. The other end of the rod has a bearing epoxied onto it. Leave a few inches of play so that the shaft assembly can slide forward and back.
On the front mount, drive a smooth framing nail within the arc of the propeller so that it can not turn. Now, open the sunroof on your car and stick this thing out on the roof. The prop can not turn because of the framing nail, yet you can slide the shaft forward a bit while holding the bearing. Measure how much force it takes to move it slightly. Now, push further forward until it clears the nail. It will begin to rotate. Again measure the force it takes to keep it in a forward position.
Be advised that this will very likely disintegrate and disfigure, dismember, and/or blind you. Happy experimenting.
BTW: I predict a spinning prop has less brake effect. It just seems like common sense to me, however I know there are scientific facts that defy logic..
That would test a free-wheeling prop, not the originally posted idling engine, where the rpm is a constant and not just getting rotated by the air coming at it. So this test would not be valid in this case. It is already established that the question is not about a free-wheeling prop.