Grumpy_Mike:
Well are discussing the techniques by which this can be done we are not talking about a universal solution.
If we stick with the 18A figure and limit ourselves to 1/2W power dissipation over the sensing resistor, the voltage drop at that power will be 0.027V.
Plug that value into Ohm's law we get a 1.5 milliohm resistor.
Assuming we can find one of those, the voltage drop across it for each LED you light up would be 0.00009V - far too small for an Arduino to measure directly.
Workable technique...?