Lepton pair production has played an important role in both nuclear and particle physics, being among the earliest calculations utilizing QED, and seen most famously in the discovery of the $J/psi$ at BNL. A technique is presented for measuring the linear polarization of GeV scale photon beams through the detection of $e^+e^-$ pairs photo-produced in the target. This technique is applied to the analysis of GlueX data on proton target. Simulation predicts the analyzing power for pair production to be $.5725 pm 0.0025$ for the GlueX data, and the analysis of experimental data gives a linear polarization of approximately 35%, in good agreement with other measurements of beam polarization. The pair production technique is complementary to other electromagnetic and hadronic measurements of beam polarization, and is generally applicable in experiments that allow for forward angle electron and positron identification and tracking. To facilitate this study, a neural net was trained for $ e/pi $ separation to eliminate the pion background. Further, it is demonstrated that these $ e^+ e^- $ pairs are sensitive to the proton charge form factor, which opens up the possibility for a new method to measure the proton RMS charge radius.