One way to test for pinholes or cracks in a cable's insulation is to immrse it in water. Connect your hipot tester high side to the cable's conductor and the low side to an electrode immersed in the water. We have also used that technique to check the encapsulation of photovoltaic panels.
I kike the idea of safety, but exactly how would a hipot test detect holes in the outside insulation of, say, a shielded cable? It seems the cable must be wrapped in foil, or passed through a conductive and compliant 'doughnut' while 'hot' to do this.