  The diffuse Galactic synchrotron emission should exhibit a low level of
diffuse circular polarization (CP) due to the circular motions of the emitting
relativistic electrons. This probes the Galactic magnetic field in a similar
way as the product of total Galactic synchrotron intensity times Faraday depth.
We use this to construct an all sky prediction of the so far unexplored
Galactic CP from existing measurements. This map can be used to search for this
CP signal in low frequency radio data even prior to imaging. If detected as
predicted, it would confirm the expectation that relativistic electrons, and
not positrons, are responsible for the Galactic radio emission. Furthermore,
the strength of real to predicted circular polarization would provide
statistical information on magnetic structures along the line-of-sights.
