I believe I would loosely hold a small object in the glow plug hole that exceeds the length of the replacement glow plug by approximately 1/4 inch and have someone turn the engine over at least one complete revolution. If the piston does not touch the object you are holding, you can safely assume you have adequate clearance for the replacement glow plug.
Make sure the fuel is turned of to insure the engine will not start. In the interest of a safe experiment, I would recommend that you remove all the glow plugs to insure that your engine does not start. You may also want to wear safety goggles to prevent any fuel mist residue from your eyes that could remain in the combustion chamber.
Maybe some other folks have a better idea.