A hot wire from the battery and quickly touched to the top of the GP will tell the story..no spark= No good. (not for the "looper" type) Thats a quick check if your on the road or dont have your tool box. I have never set a car on fire by doing this or has the old mechanic that showed me this quick check.
The best check is to pull them out (I pull the hard injector lines also) and use that hot wire and a ground wire and see if they quickly get red hot.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"