Another way is bi-static radars. Stealth is generally designed to prevent returns from going back to the sender, with the downside that they get scattered in all other directions. Just setup a (silent and difficult to locate) receiving station(s) and give it a communication channel to the sender (to synchronized), then it will receive the emissions from the sender that bounce off stuff.

Another way of using bistatic radar is to look for holes. Pick a frequency band that has significant background noise and search for places where the background is consistently lower than average. Essentially, invert the detection algorithm. Nice thing about this approach is it can be done non-cooperatively; that is, the EM sources do not need to be controlled by whomever is running the receiver.