Many FABO members can ""read" the plugs to see how they are burning. This is how many people determine if you need hotter of cooler plugs. See example below:
This is why reading plugs can be a bit tricky. The plug on the left doesn't look lean to me. That plug is way too hot and you can see the fuel on the plug that is being thrown in there to cool that plug.
This is also why an O2 sensor can trick you. I'd bet a everything I have that the A/R with that plug and tune up is what most would consider "normal" or correctly tuned, when in reality it is dead rich with a hot plug and the O2 sensor doesn't know the difference.