You want the propeller stopped for best glide in a power plane with a fixed pitch prop. A windmilling prop is a powerful speed brake. On the other hand, if an engine quits at low altitude, trying to get the propeller stopped might be too dangerous. But it would be useful to know the values for both cases.
The propeller spinning with the engine at idle will be reasonably close to the drag of a stopped prop, which is significantly less than a windmilling one, and more than one turning at a "zero thrust" speed (somewhere off idle, but no way to know exactly where that is without the ability to measure thrust at the engine)
Just get it stabilized and trimmed hands off in a glide in the configuration you want, in smooth air, engine idling, and note VS and TAS. Convert the TAS to Ft/Min, or the VS to knots, divide one into the other, and there's your glide ratio. If you want more precision, do a timed descent using the altimeter instead of trying to discern the VSI.
If you're feeling brave, you can get to a safe altitude over your airport or some other uncontrolled airport with a decent long runway, and shut the thing right down at altitude, and take your measurements with the propeller windmilling and stopped, restarting as soon as you're done, and allowing for the engine to warm up.
You might have to slow down close to stall to get it to stop, depending on how good the engine's compression is (a newer, tight engine may stop turning at or above best glide speed). If you had an engine failure at high altitude and needed to plan a long glide and needed every foot of distance because your engine quit over a glacier or a field of boulders, it might be worth the effort to get the prop stopped.