If you have not already seen it The Diesel Giant site has a pictorial on checking the Glow Plugs with a meter. When the page opens up it appears that there is not much there; you have to scroll down to see the pics.

If you remove the Glow Plugs the carbon should be reamed or otherwise cleand out of the Glow Plug Holes.
Totally Avoid Autolite Glow Plugs and Monarks. Use Bosch or Beru Glow Plugs. Bosch are good, cost less and easier to find.

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84 300D, 82 Volvo 244Gl Diesel