BMW Repair: bmw 318i 52 plate Brakes, bmw 318i, warning light

Question
i have a 52 plate 318i saloon. i have had an amber warning light come up (brake pads). 3 questions. how do i know whether front or back pads ? how much wear is there left on pads ? i assume that you dont normally have to change discs also as a matter of course, i know some garages that will try it on ?
40,000 mls
thanks

Answer
The sensors for the brake pads are located on the left front wheel and the right rear wheel so checking it out involves only removing 2 wheels.My first guess would be the front as it gets the harshist braking effort.If the rotors are worn beyond spec they will have to be replaced.With only 40k I would assume that the rotors maybe able to be resurfaced(turned)BVut I would have them measured FIRST.
The sensor sets the light by 2 ways,if the it is open or grounded,check LF and RR wheels