Turn off the power!
If you don't know which cable is Supply and which is Load then pick either cable.
Connect Live and Neutral to L1 & N1 and put the other cable in L2 & N2 Live & Neutral.
If the wires are correct with respect to Load & Supply the Neon light will go on and off as you operate the switch.
If the wires are back to front - Load and Supply crossed over - the Neon will be on all the time. Just change the cables around.
Good luck...Take care...
Peccavi
March 2009