I copied what appears below. The person who wrote this is very experienced with Porsche cars.

To "read" spark plugs, one must perform a proper plug cut, that is; switching off the engine after being held at a specified RPM & throttle position for 10 seconds or more.

Keep in mind that unleaded fuels read differently than the old leaded ones and a normal center insulator colors up (under load) a very light tan, reddish brown, or almost white, depending on the brand of gasoline. One big thing about reading plugs is observing the positon of the fire ring on the insulator. This tells the reader how hot peak combustion temps are.

It takes a fair bit of experience to accurately read and interpret spark plugs, but its quite accurate when you know what you are looking at....