During taxi / pre-take off I see a lot of commercial airliners test the deflections of their various control surfaces like the rudder, ailerons etc.
What's the feedback received in this operation i.e. The pilots themselves cannot visually verify the full deflections achieved on command, right? Unlike on (say) a carrier etc where there's a spotter on the ground to confirm.
Just curious, what is the failure mode to be revealed in this sort of test. Is there some sort of internal avionics indicator that would tell the flight crew if the control surface was not reacting to the inputs as expected?