The direction of the inner-edge polarization of a halo can serve as an observational diagnostic for determining the actual nature of a halo arc if two competing explanations exist. The observation can be decisive for the identification of a spot that might be either a 44° parhelion or a 46° parhelion, of an arc that might be either a 22° sunvex Parry arc or a 20° Parroid arc arising from plate-oriented pyramidal crystals, and of an arc that might be either a 22° suncave Parry arc or a 23° Parroid arc from plate-oriented pyramidal crystals. Practical hints are given for observing visually the inner-edge polarization of halos.