Simply by removing the road wheel, you will be able to see the remaining pad thickness. Outer pads can be seen without removing the wheel however inner pads are more difficult to check.
A couple of points to remember about maintaining decent pad depth;
1. You are pulling up a vehicle weighting over two metric tons ( 2000 kg +)
2. Brake efficiency drops off as the pad reaches minimum thickness due to heating and reduce material mass
3. You will, due to normal brake bias, burn up fronts at about twice the rate of rears, however don't assume that the rears don't need inspection
4. New brake pads ( and even rotors ) are cheaper that panel work !!!
Remember whilst braking, a massive rush of adrenaline is far more enjoyable than massive panel job.
2008 V6 TDI, Air Suspension, ACC with Front Assist, Paddle Shifter, RNS510, Leather Seats, BI Xenons, 4 Zone Climatronic, Sunroof, Towbar & Trailer Module,Fiscon Bluetooth. Reversing Camera, Front Fog Lamps Xenon 3000K, Redarc TowPro Brake Controller, T1 Door Warning Lamps, T1 Under Seat Drawer, Alloy Skid Plate, 5ton Winch, Cayenne (Green Dot) Front and Rear Sway Bars, Performance Tune by Oettinger, Retrofitted 6 Piston Brembo Front Brake Callipers, Custom Sports Steering Wheel