  The magnetic field in our Galaxy is not well known and is difficult to
measure. A spiral regular field in the disk between the Galactic arms is
favored by observations, however it is still controversial if the field
reverses from arm to arm. The parity of the field across the Galactic plane is
also not well established. In this letter we demonstrate that cosmic ray
protons in the energy range 10^18 to 10^19 eV, if accelerated near the center
of the Galaxy, can probe the large scale structure of the Galactic Magnetic
Field. In particular if the field is of even parity, and the spiral field
reverses direction from arm to arm, i.e. if it is bi-symmetric (BSS), ultra
high energy protons will predominantly come from the Southern Galactic
hemisphere, and predominantly from the Northern Galactic hemisphere if the
field is of even parity and axi-symmetric (ASS). There is no sensitivity to the
BSS or ASS configurations if the field is of odd parity.
