One sign of heat used by some professionals is electrode color. Many--but not all--spark-plug electrodes have an alloy composition that will oxidize green when subjected to high temperatures (see Photo 2, again). This nickel oxide on the center wire and sidewire would show up first at the tip of the electrode--the hottest part--and progress from there, depending on temperature. Excessive green oxide may signal it's time to change to a colder plug. Several causes of high spark-plug temperature are described in the boxed information at left. The main thing to keep in mind here is that not all electrodes oxidize (turn green) at the same rate, if at all. Therefore, your best bet is to turn to a technician with experience reading plugs.