Way back in the olden days we did stuff like this with some sort of pulse generator on the shaft (optical, Hall Effect or magnet/coil) and a pretty simple logic circuit that compared the pulses to a time base. It could be as simple as a 555 timer set up as a missing pulse detector. If the motor pulse didn't reset it within the window it triggered the alarm.
Usually they would just suppress errors for a second or so to let the motor speed stabilize.
An even older scheme used a small DC generator on the end of the shaft. If the output voltage dipped, the motor was slowing down. That was really more of an adjustment tool than a monitor though. We used them to set up the hydraulic motor/spool valve assembly on line printers (looking at it with a scope).
