What am I looking for with strobe disc?

Forgive me,I thought you were looking for a unit.
When on 33 rpm,the 33 marked portion should be at rest and stable.If it drifts,then an adjustmentment is in order.Provided the table has such a device.
Seans' description is right on target.My observations are referenced to the KAB unit.

Thanks much for the info! I think I'm getting the hang of it. The PLC on my VPI is MUCH better than without, go figure.

Related questions: Clearaudio has a strobe disk with a drag groove which allows one to measure the speed with the drag created by the stylus. Is the drag created by the stylus significant? Will it vary from record to record (or even track to track (or even moment to moment as the pitch of the music changes))? Will the drag vary depending on the weight of the platter? Is the industry standard 33 1/3 assuming no stylus drag? Are we all nuts to worry about this?

The "strobe light" can just be an inexpensive neon circuit tester. Any light source that flickers with the power line frequency. Fluorescent lights used to work, but I think they have improved them so they don't flicker these days.

The effect, if any, of stylus drag will depend on your particular turntable: platter mass, motor drive power, etc.