  The composition of the relativistic plasma produced in active galactic nuclei
and ejected via powerful jets into the interstellar/intergalactic medium is
still a major unsettled issue. It might be a positron-electron plasma in case
the plasma was created by pair production in the intense photon fields near
accreting super-massive black holes. Alternatively, it might be an
electron-proton plasma in case magnetic fields lift and accelerate the thermal
gas of accretion discs into relativistic jets as the recent detection of
$\gamma$-rays from blazars indicates. Despite various attempts to unambiguously
establish the composition of the relativistic jets, this remains a major
unknown. Here, we propose a way to settle the question via sensitive
measurements of circular polarization (CP) in the radio emission of the hot
spots of bright radio galaxies like Cygnus A. The CP of synchrotron emission is
determined by the circular motions of the radiating relativistic leptons. In
case of charge symmetric energy spectra of a electron-positron plasma, it
should be exactly zero. In case of an electron-proton plasma the electrons
imprint their gyration onto the CP and we expect the hot spots of Cygnus A to
exhibit a fractional CP at a level of $10^{-3}\,(\nu/\mbox{GHz})^{-{1}/{2}}$,
which is challenging to measure, but not completely unfeasible.
