The best way to check the glow plugs (GP) (except for the old style series loop GP) is to remove them and connect them directly to the battery. They should glow bright red in about 10 seconds or less.
The best way to do this is to connect a #12 or heavier wire to the GP terminal and clamp the GP to a good ground with a Vice Grip. Then touch the other end of the wire to the hot side battery terminal. If the GP doesn't glow red hot in 10 seconds or less, it is NG and should be replaced.
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