An easy start is always looking at VHF reflection of strong transmitters that are not creating a lot of noise (like FM stations). ILS or VORs stations are classics.
There are also a lot of meteor and space radars ( https://en.wikipedia.org/wiki/GRAVES_(system) ). Universities are super happy in publishing very very detailed (https://www.iap-kborn.de/en/research/department-radar-remote...) specs of their sky radars. And frequencies are well known for ages.