How do most folks know when a ball coupler is worn out? The same way they know when the wooden deck of their trailer is ready for repair... when it fails. In one case the tractor is astride the axle(s) and in the other you have a trailer testing your safety chains and tempting fate.
Grease is cheap. Put some on the ball and it will get on the coupler. Every once in a while use a throw away rag to wipe the dirt/grit and grease mixture out of the coupler and off of the ball and re-grease.
I don't know what is peanuts to some folks but I buy very high rated hitch components (e.g. balls rated OVER 11,000 LBS) and my drawbars are solid square stock not tubing. I think a few cents worth of grease once in a while to extend their life and the life of the coupler is not foolish extravagance of time or materials to preserve cheap throwaway components.
Pat