I guess you could inspect the plugs, but if I was going to that trouble I would surely just replace the plugs. I do mine every 30,000 miles. Be sure to get the proper non-resistor type plug. Bosch F8 DC4's. Many threads on this topic. Do not use the platinum plugs. Remember to use compressed air or a computer canned air to blow any crud out before you remove the spark plug(s).

A known issue are the rubber connectors, 3 of them. They deteriorate over the years due to the heat. I just replaced them on my '97 S320. I don't think visually inspecting the coils is useful. You can find them on line.

Steve