Always forget to mention this because I do this without thinking. Practically all the plugs are of the resistor, carbon resistor is embedded in the upper part of the ceramic.
With an ohmmeter check the resistance between the top connector and center electrode, should be 8,000 ohms, if much more, or can even have an open circuit, part of the spark would occur in that resistor connection, and not across the electrodes where it belongs. Just as important as getting the proper gap.
Another test requires a spark plug tester, sealed area where you can increase the pressure to 150 psi, may be find you have blowby between that ceramic and metal base. Is nice to have this basic equipment, your engine is the last thing you want to use to even test brand new plugs.
Was really unhappy when I drove my brand new Cruze home, very particular, was having misfires. Gaps were way off, springs were hung up in the boot shoulder.
Debate on whether to put a very thin coating on the threads, its your vehicle, wreck if you want, been doing this before most of you were even thought of. Even with some plugs where the threads stuck out into the combustion chamber, put those in my lathe can removed those worthless threads. Also dielectric grease in the boots, even with the spark plug wire type, if you don't they will bake on.