  We show that measurements of the proton's polarization in electron-positron
annihilation to a proton-antiproton pair $e^+ e^- \to p \bar p$ strongly
discriminate between the analytic forms of models which fit the proton form
factors in the spacelike region. In particular, the single-spin asymmetry
normal to the scattering plane measures the relative phase difference between
the timelike $G_E$ and $G_M$ form factors. The expected proton polarization in
the timelike region is large, of order of several tens of percent.
