For proper operation of the LCLS [1] x-ray free-electron laser (FEL), and other similar machines, measurement and control of the electron bunch longitudinal phase space is critical. ... We propose a diagnostic scheme by exciting a weak skew quadrupole at an energy-chirped, high dispersion point in the first LCLS bunch compressor (BC1) to reconstruct longitudinal phase space on an OTR screen after BC1, allowing a time-resolved characterization of CSR effects.