Detection of "constant" signal
Perhaps transferable to your problem, one method from Olyha B., Rutledge J., "TrackPoint System Version 4.0 Engineering Specification", IBM, 1999: 


  2.2.3 DRIFT CORRECTION
  
  Due to the significant temperature sensitivity of the force sensors, it is necessary to recalibrate the zero force
  origin of each axis on a periodic basis. In order to properly
  recalibrate, it is necessary to determine when a ‘hands off’ condition
  exists (i.e. when no force is being applied). This is done by
  monitoring the X and Y axis force samples, and when there is minimal
  change in the readings (no more than ±1 count) for a specified length
  of time (3 seconds by default) it is assumed that the hands off
  condition exists. At this time the X, Y, and Z axis origins are reset
  to the smoothed current value of the X, Y, and Z axis force samples
  respectively. It should be noted that noise introduced by any source
  other than applied sensor forces will severely impact the
  effectiveness of this technique.


In my experience as a long-time ThinkPad user, this technique cannot always differentiate between the user trying to move the mouse cursor at constant speed and real sensor drift. Then again this is not usually dangerous and it is easy to manually trigger recalibration by hands off. Perhaps there should be a more advanced analysis algorithm running that can identify velocity variation in a manual attempt to move the mouse cursor at constant speed.