Another way is to put an ohm meter from the plug to ground, without battery on it, you should get some resistance, it was mentioned .08-1.2 Ohms,Interesting.
IMHO, if the plugs don't work, you should make sure that power is getting to the plug before removing it. I have seen more cases of bad wiring and dirty connections than I have of bad plugs.
Have you tested the plugs you removed to see if they work when connected directly to 12V?
A real quick test is to run a HEAVY wire from the positive lead on the battery to the tip of the plug and THEN see if the plug gets hot.