Sense 230V - current
Current sensor with hall effect solves both detection and insulation problems.


  I don't need to know which winding is absorbing


However, in your other question:


  in this project I need to monitor the voltage on the two wires, in order to see whether a command is sent


So, why don't you use two hall effect current sensors on two lines and kill two birds with one stone - get current and direction simultaneously.

This also takes care of your third stated goal of minimal space.