We report a new, to the best of our knowledge, method for detection of dynamic population gratings in Er-doped fibers with saturable absorption that is based on observation of the transient fluorescence resulting from fast displacement of the recording interference pattern. The signal appears due to essentially different relaxation times of the fluorescence in bright and dark fringes of the light pattern. The experimental results obtained at the recording wavelength $\lambda =1492\mathrm{nm}$ in a single-mode fiber with $5600\mathrm{ppm}$ Er demonstrate good agreement with the theoretical model based on a saturable two-level system. Unlike conventional diffraction-based techniques, the proposed method results in the local grating amplitude value and can be used for characterization of the grating amplitude spatial profile. It can also be suitable for characterization of the grating formation processes in other bulk and thin-film media with saturable absorption.