Use the Laws of Physics

In order to get light to escape from a length of fibre optic, it has to be bent through quite a sharp angle (determined by the refractive index of the material).

Why can't fibre manufacturers formulate their materials a little more carefully, so that the fibre will break -- thus notifying the intended recipient that something is amiss -- at a rather shallower angle than that at which light is allowed to escape?

A dye could be added which will turn opaque at high temperatures, so as to thwart any attempt to prevent such breakage by softening the fibre with a blowtorch.