Nothing directly monitors the tail skid position. The SPSEU takes discrete inputs and when certain conditions are met powers the solenoid in the tail skid actuator and extends or retracts it. There is no position feedback to the SPSEU.
The SPSEU light can be triggered by internal fault, discrete input faults or tail skid output overcurrent. So, if you get a SPSEU light and don't have the mid cabin doors, you can be sure the tailskid is inoperable.