Is there any (relatively simple) way to determine whether a stepper is being directed to move by the voltage signal across its winding?
So it would seem that there is always a pulsing waveform across the windings whenever the stepper is energized, correct?


Correct.


  When the motor is holding its position, is there anything distinctive about the waveform?


Yes there is. This video contains a short example of what the current waveforms would look like.

If I understand you correctly: You want to detect filament events based by doing current sensing on the phases of the stepper motor. The simple of it is that, if the motor is not turning then the Back ElectroMotive Force will be zero. As the speed increases the BEMF goes up, which causes the current levels to go down. Is this how stall detection in some driver systems work.

You may be able to determine if what you want to do is even practical by studying the fundamentals of stepper motor driving; both voltage mode and current mode.

Apart from that it may be just as practical to use a rotary encoder upstream of the extruder motor to detect the filament movement, or lack thereof.

In your case detecting if the filament is present but not moving calls for stall detection. If the PWM pulse occurs but the rate at which the voltage increased (the slope) did not change, then there was no BEMF event. The lack of such an event would indicate that the rotor did not move. If there is no filament to push, then when the rotor moves it will ring (overshoot it's target position and fluctuate back and forth until it settles into the electrical position that the controller intended), because the mechanical load has changed.

For a NEMA17 stepper, these would be very small change in a very brief event. I would start with installing current sensors on both phases, and then doing some signal processing on top of that.