Is it possible to determine load on a stepper motor when stopped
I am not that interested in the precise value of the torque, I am more
  interested in checking that there is no significant torque.


Since you don't need the precise value of the torque, perhaps stall detection or torque limiting or both will be adequate for your application.

torque limiting

The maximum torque that a stepper motor can produce is proportional to the current through the motor coils.

Many stepper drivers make it easy to set a limit on the maximum current through the motor coils.

Perhaps in your application, it would be sufficient to simply dial down that limit so the motor never applies "significant torque".

stall detection

Many stepper motor drivers such as the
Trinamic TMC249A, Trinamic TMC246, TI DRV8711, ST L6470, ST L6482, ST L9942, ON AMIS-30623, Allegro A4979, etc.
have "sensorless stall detection".


"How to detect when a stepper motor has stalled?"
"Back EMF method detects stepper motor stall: Pt. 1"
"Back EMF method detects stepper motor stall: Pt. 2"
Application note AN3327: L9942 back EMF stall detection algorithm
"A robust method for stepper motor stall detection"
"Back-EMF method detects stepper-motor stall"
"Innovative back EMF based stall detection simplifies stepper-motor drive designs"


As Dave Tweed already stated, the back-EMF is proportional to the speed of the motor.
My understanding is that these "sensorless" techniques rely on directly or indirectly measuring the back-EMF while rapidly stepping the motor.
So these techniques don't detect anything while the stepper driver is holding the stepper motor in one position (or trying to drive it at a slow speed).

As you suggested, "jogging the stepper backwards and forwards a step and watching the current may be plausible."
While the stepper driver is rapidly stepping the motor forwards and backwards a few steps,
then the stall detection circuit can work:
If the back-EMF is zero (or below some threshold), then the motor has stopped (or the speed of the motor is below some threshold);
if the back-EMF is above the threshold, then the motor is moving at least some threshold speed.