A good spark plug will have no build ups or deposits on it and no worn or damaged spots. Look at the colouration of the ground strap to see if the heat range is accurate. Any colour change near the end of the strap indicates that the heat range is too cool, that the heat is being absorbed too quickly to provide a good burn. Colour change that occurs too close to where the strap connects to the base reveals a range that is too hot. The colour transition should occur half way down the ground plug. One of the threads on the plug might also show some discolouration.
I've already had a Google and read about reading a plug I was hoping for someone in the know really. I'll get it booked in for a r/r to have checked.
Cheers