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.
1. Let's stop re-explaining how radar works as an argument, everyone gets it. For those who don't - radar works like turning on a flashlight, end of story.
2. Non-SEAD RWR is generally limited to "There is light shining on me." Not where it's coming from, let alone providing targeting information.
All modern (1980s and later) RWRs can determine the bearing of emissions. Heck, even the RWR in the Mig-21 (1950s plane) could determine bearing. With bearings known, all you need is two planes to determine location by taking the intersection of the two bearings. Most RWRs can also identify the type of radar, as well as whether it is searching or locked into a target. Planes can also equip targeting pods that have even more advanced passive sensors: https://en.m.wikipedia.org/wiki/AN/ASQ-213
