  Recent measurements of the azimuthal anisotropy of direct photons in
heavy-ion collisions at the energies of RHIC show that it is of the same order
as the hadronic one. This finding appears to contradict the expected dominance
of photon production from a quark-gluon plasma at an early stage of a heavy-ion
collision. A possible explanation of the strong azimuthal anisotropy of the
photons, given recently, is based on the presence of a large magnetic field in
the early phase of a collision. In this letter, we propose a method to
experimentally measure the degree to which a magnetic field in heavy-ion
collisions is responsible for the observed anisotropy of photon production. The
experimental test proposed in this letter may potentially change our
understanding of the non-equilibrium stage and possible thermalization in
heavy-ion collisions.
