The best way is to monitor the alignment voltage. Please do not reset the DS240/241- doing so will recalibrate the beam smokes and will give wrong readings.
Once trouble is detected, read the alignment voltage at the display plate or at the receiver (the access cover must be removed). Use the TC2000 test cable.
Check for fluctuations in voltage, it should remain steady (+ or -) .25VDC
If voltage decreases or increase over a period of time (give an hour) - then there is possible movement or environment changes.
Another suggestion is to mount a laser pointer near the beam smoke on the wall. Mark where it hits the other wall leave it for 2 to 3 hours or longer and see if the laser point moves from its original mark.