Modern radio surveys provide new opportunities to reliably identify and study different classes of active black holes, provided that we use more information than just the radio flux at one frequency. A combination of the radio spectrum and morphology, or cross-matching with high-frequency data, can be used to robustly select AGN at different redshifts. This is demonstrated with two examples. Firstly, we have compiled the first complete sample of low-power radio galaxies (eg, Centaurus A's and M87s) in our local universe. These tentatively show a significant correlation with the arrival directions of ultra-high energy cosmic rays and we find clear evidence that today the large scale environment is causally related to the production of powerful jets. Secondly, we show that FRIIs (eg, Cygnus A's) are a good tracer of black hole activity through cosmic history as they are easy to identify in any radio survey with sufficient angular resolution. We use the FIRST survey (1.4 GHz) to obtain a homogeneous sample of hundred thousand FRIIs. Using SDSS in addition, a tight correlation between disk power and lobe radio luminosity is observed, indicating that the physics of jet-disk coupling is identical at low and high redshift. Interestingly, we measure a significant over-density of FRIIs, compared to what is expected from X-ray, optical, and near-IR quasar surveys. These "diskless jets" from high-power active black holes (L>1045 erg/s) could be explained by: extreme obscuration (at all wavelengths), radiatively inefficient accretors, or jets that are powered only by the spin of the black hole. Our results are also relevant for near-future surveys by SKA pathfinders and we show some promising first results by applying our method to LOFAR commission data.