Detected by Radar Warning Receivers (RWR). These are passive sensors, they're listening to the radar emissions that other planes and Radar stations are emitting. Because the radar needs to send a signal that bounces off the plane and have enough energy for those bounces to travel back to the radar, whereas th RWR only needs to detect the emissions traveling from the radar to the plane the radar will be detected by RWR from much further away. Most modern aircraft carry RWR, not just AWACS.