You can also get an idea of plugs' condition testing with an ohmmeter or test light. Good plug will have relatively low resistance to ground, dead plug will be open. Usually this is an accurate test although not always, however, gives you some idea at least. Only trouble is that you have to detach the bus bar from all 6 plugs in order to test accurately, with them all hooked up this test only tells you whether or not they are *all* dead.
Or, if you have access to an ammeter with fairly high current capacity, you can check current thru the supply wire from the GP relay and get a pretty close idea of how many plugs are working and how many are dead (though you won't know which ones are which). Each plug will draw about 12 amps after a couple seconds. Fully functioning system should give you 72 amps, getting 48 amps means only 4 are live, 24 amps means 2, etc, etc. A little inductive meter that you hold next to the cable (e.g. the type sold for checking alternator charging current or starter draw) works reasonably well for this.
I pulled some plugs out of a 6.9L Ford once that was getting ether every morning. The tips had exploded. Pretty violent event when that stuff goes off and if the plugs are hot at the time, it's not good. A last idea would be to pull out a few of the easily accessible plugs on your motor (cyls 1-4). Appearance of what comes out of there might give you some clues as to what's keeping it from lighting off.
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