In a gasoline engine deposits or discoloration on spark plugs can be used to diagnose problems, for instance lean or rich running, or ignition problems. There are many handy pictures available to show the details of these conditions.
Glow plugs, while they only operate for a short time before starting a diesel engine, still stick out into the pre-combusion or combustion chamber, so are still exposed to the combustion. Can the condition of glow plugs be interpreted for diagnostic purposes?