If you don't think measuring the resistance to ground is adequate then you can test the plugs in the block. Just pull the injectors and sealing rings. Turn on the ignition and look in the prechambers. Watch the glow flies come to life. It's amazing how hot they get. Make sure you have spare seals around and at least a metre of drain line. I usually do this when I inspect injectors for crack/reseat/spray, replace filters and check timing.
All the posts about not needing the GP's in the summer are correct unless the injectors are clogged and not spraying correctly. In this case the GP's help an out of tune engine get started. The same thing could happen if the timing is advanced too far. The air in the cylinder would not be hot enough to cause ignition when the fuel enters the cylinder. Sure sounds like you need to forget the GP's and move on. Good Luck.