A simple feeler gauge check for gap between the mechanism, bolt, nut etc, is all that is required on the scissor shackle.
Actually, a conventional feeler gauge would't work, as the two surfaces are not in proximity, with the scissor shackle closed. As the bolt is tightened, the opposing faces of the 'jaws' of the drogue shackle do not remain parallel. The gap at the open end becomes smaller than that at the inboard end.
You are right though in saying that some sort of gap measuring device would do the trick.