1. WOW sensors are usually of the simple contact proximity sensor.
2. There are at least 2 WOW sensors in each aircraft. If all fail at the same time(very rare), most a/c's NWS steering would be inop. In such cases, the pilot simply stops the a/c on the runway and requests for towing assistance. The A380s quite often had this problem with a faulty BSCU.
Most a/cs WOW sensor is placed on the NLG. Some on the MLG. The a/c estabilishes it is in ground mode once the oleo/shock strut starts to get compressed. So, for a worn out shock strut or a decelerating plane on the runway, the a/cs shock strut would still be compressed letting the a/c know it's in ground mode.
You can see when a 747s go into ground mode simply by looking at the nose wheel light. It only lights up when the a/c is in ground mode.