Temporal phase shifts induced by cross-phase modulation in an optical fiber are directly characterized with a spectral equivalent of the Foucault technique used to spatially resolve wavefronts. The temporal phase induced by a high power pulsed pump on a monochromatic probe via cross-phase modulation is converted in a temporal intensity modulation via spectral filtering. A measurement of the modulated instantaneous power of the filtered signal allows to directly determine the time-resolved nonlinear phase shift. Additionally, an equivalent of the transport-of-intensity equation, which links the evolution of the instantaneous power of the electric field in a dispersive medium to the instantaneous values of the power and phase of the field. This derivation permits the measurement of temporal phase shifts using only intensity information in a direct, non-interferometric manner.