This project aims towards probing the relativistic particle content of the most numerous, persistent population of high energy cosmic sources in the Universe: jets of active galactic nuclei (AGN). This undertaking is leveraged by today's synergy of high performance ground- and space-based photon detecting instruments as well as key advances in astroparticle physics experiments, yielding dense high quality data over a broad energy range from a large collection of AGN. Such powerful objects are among the prime candidate sources of the enigmatic ultrahigh energy cosmic rays, however, conclusive evidence is still missing. We aim to develop diagnostic tools that probe the composition of the material in AGN jets, and allows a systematic search for observational features of hadronic interactions from high energy cosmic jet sources.