The Muon g-2 experiment is completing our final analysis of the muon anomalous magnetic moment, with our largest dataset to date. The electric field correction to the anomalous precision frequency is our largest correction and remains a critical systematic. This correction is determined from the muon momentum. Using a new CENPA-built in-beam detector, we directly mapped the momentum distribution as a cross-check for the established reconstruction methods from indirect detectors. The comparison shows overall good agreement, with subtle discrepancies which we understand through simulation and are working to improve.