Quantitatively measuring nozzle wear
Based on CNC Kitchen measurements,

                
            
the wear can be initially estimated by judging the overall length of the nozzle. Abrasive particles wear the nozzle very little on the channel (so the diameter doesn't change, for a while) but they wear out the outer surface in contact with the printed part, resulting in a shortening.
Of course, once the shortening gets severe, the diameter will increase because you reach the inner chamber.