Shure sold a microscope setup for assessing stylus wear. Their 'scope was 200X, with two side lights to illuminate the stylus' surfaces that contact the groove. The manual that came with it had photos of new, slightly worn and worn out styli for each different type of stylus (conical, elliptical, etc.). You compared the photos to the stylus being examined. Even with this equipment, it took some experience to properly assess stylus condition.
I just finished reading a good article on stylus wear in a 1980 "Sound Canada" magazine. The writer used to work in the pressing plant of a major record manufacturer. He said that the quality control people could replace styli on test turntables when they felt it was needed. He examined rejected styli using good equipment, and often could not determine any wear. So he did some blind testing with QC people, and they could tell a new stylus from a rejected stylus without fail. His conclusion was that the best equipment and an experienced tech wasn't as reliable as experienced ears when it comes to identifying stylus wear.
