It's not possible to ascertain wear on the diamond with these tools, but we can get important and useful clues and hints to the care (or lack of) the stylus received over the last 40 years. For example, playing dirty records eventually creates spatterings of tiny dirt particles around the metal and plastic parts of the stylus, which a loupe or "pocket microscope" will reveal. You can also see if there's crud on the tip of the diamond.
To check for wear, however, simply playing a record with female vocals containing a lot of S sounds will tell the tale. If the S sounds turn into SH sounds (or, worse, SXX sounds), and you're tracking this stylus up around 1.75g, chances are it's worn. Fingers crossed that it sounds good, because it's really an excellent stylus, considering that it was Audio-Technica's poor man's Shibata of the time.