  Magnetic fields in various astrophysical settings may be helical and, in the
cosmological context, may provide a measure of primordial CP violation during
baryogenesis. Yet it is difficult, even in principle, to devise a scheme by
which magnetic helicity may be detected, except in some very special systems.
We propose that charged cosmic rays originating from known sources may be
useful for this purpose. We show that the correlator of the arrival momenta of
the cosmic rays is sensitive to the helicity of an intervening magnetic field.
If the sources themselves are not known, the method may still be useful
provided we have some knowledge of their spatial distribution.
