Presently the Belle II experiment at SuperKEKB is colliding $e^+e^-$ beams at the $\Upsilon(4S)$ resonance. These beams currently have no polarization, but if SuperKEKB and Belle II were to be upgraded to make use of polarized electron beams a significant number of electroweak precision measurements could be made. However, in addition to the technical difficulties in creating a polarized beam for collisions, it is difficult to know the exact amount of polarization that remains at the moment of collision. This uncertainty can become a leading systematic uncertainty limiting the precision of physics measurements.
The beam polarization can be measured with sub-percent precision by making use of the relationship between beam polarization and the forward-backward asymmetry in the polarization of tau leptons produced in the $e^+e^-$ collisions. By measuring the asymmetry, a precise value for the beam polarization at the e+e- interaction point can be determined.
In this talk I will show results from applying this analysis method to the unpolarized beams delivered by SLAC to the BaBar experiment. I will also discuss the feasibility of applying the analysis technique as a measurement tool of beam polarization for a potential upgrade of polarized electron beams to SuperKEKB/Belle II.