There is no real way to see if the insulator is bad without disassembling the plug. Yes deep inside the metal the plug ceramic insulator can be cracked. I have done this just to prove to myself what I was thinking was happening on some engine where the plug would fire outside the compression zone but not when under compression load.
Of course this will destroy the plug. Just easier to test with a known good plug. Even new plug can be bad right of the box. And just accept the results especially under shop conditions where time wasted is not good for the bottom line. When shop rates are around $65 or higher a $3 plug can cost a lot more.
But during quite times learning what was the problem does help in future troubleshooting. Here the 10mm NGK CMR6 plug has such a high failure rate that I replace them with Bosch plugs which rarely see a failure with.
