Quoted from phishrace:The first place dirt will show is in the inlanes and just above the flippers at the bottom of the slings. When you see ball trails in the inlanes or dirt above the flippers, it's time to clean. If you don't actually see any dirt, don't clean it.
Yep watch them inlanes. To go a little further...you don't have to be super anal aboot cleaning and do it all the time like some people might say.
I've also never used any carnuba wax on any game I've owned.