Methods for locating fluid interfaces in a cased wellbore include generating vibrations in the casing, thereby forming oscillations in the wellbore fluids and the casing. The oscillations are detected by a vibration detector. The oscillations are recorded by a data acquisition system. Mathematical processing of the oscillations by cepstrum analysis is performed to determine the depths of interfaces between fluids in the annulus. The methods may also be employed to determine the time at which a cement slurry begins to set and harden. The methods may be performed in real time.