  A leading candidate for the extragalactic source of high energy cosmic rays
is the Fermi engine mechanism, in which protons confined by magnetic fields are
accelerated to very high energy through repeated scattering by plasma shock
fronts. In the process of acceleration, collisions of trapped protons with the
ambient plasma produce pions which decay to electromagnetic energy and
neutrinos. For optically thin sources, a strong connection between the emerging
cosmic rays and secondary neutrinos can be established. In this context, we
show the feasibility of using the Glashow resonance as a discriminator between
the $pp$ and $p\gamma$ interactions in Fermi engines as sources of neutrinos.
In particular, we demonstrate how three years of observation at the km$^3$
IceCube facility can serve as a filter for the dominance of the $pp$
interaction at the source.
