  With the discovery of Kuiper Belt binaries that have wide separations and
roughly equal masses new theories were proposed to explain their formation. Two
formation scenarios were suggested by Goldreich and collaborators: In the
first, dynamical friction that is generated by a sea of small bodies enables a
transient binary to become bound ($L^2s$ mechanism); in the second, a transient
binary gets bound by an encounter with a third body ($L^3$ mechanism).
  We show that these different binary formation scenarios leave their own
unique signatures in the relative abundance of prograde to retrograde binary
orbits. This signature is due to stable retrograde orbits that exist much
further out in the Hill sphere than prograde orbits. It provides an excellent
opportunity to distinguish between the different binary formation scenarios
observationally.
  We predict that if binary formation proceeded while sub-Hill velocities
prevailed, the vast majority of all comparable mass ratio binaries have
retrograde orbits. This dominance of retrograde binary orbits is a result of
binary formation via the $L^2s$ mechanism, or any other mechanism that
dissipates energy in a smooth and gradual manner. For super-Hill velocities
binary formation proceeds via the $L^3$ mechanism which produces a roughly
equal number of prograde and retrograde binaries. These predictions assume that
subsequent orbital evolution due to dynamical friction and dynamical stirring
of the Kuiper belt did not alter the sense of the binary orbit after formation.
