Systems and methods for measuring the stress profile of ion-exchanged glass are disclosed, based on the TM and TE guided mode spectra of the optical waveguide formed in the ion-exchanged glass. The method includes digitally defining from the TM and TE guided mode spectra positions of intensity extrema, and calculating respective TM and TE effective refractive indices from these positions. The method also includes calculating TM and TE refractive index profiles nTM(z) and nTE(z) using either an inverse WKB calculation or a fitting process that employs assumed functions for nTM(z) and nTE(z). The method also includes calculating the stress profile S(z)=[nTM(z)−nTE(z)]/SOC, where SOC is a stress optic coefficient for the glass substrate. Systems for performing the method are also disclosed.