Time-resolved ground-state bleach and excited-state stimulated emission spectra are measured for a cyanine dye, 3,3'-diethylthiatricarbocyanine iodide, dissolved in methanol by spectrally resolving a background-free three pulse four-wave mixing signal. It has been shown from model theoretical calculations and experiments that dynamics of the nuclear wave packets created in both ground and excited electronic states can be observed through the oscillations of the spectral line width as well as the center frequency in time. For a system with small Stokes shift, where the ground-state bleach and the stimulated emission spectra cannot be resolved into two separate bands, unambiguous assignment of an oscillation in the whole spectra to either the ground or the excited electronic state could be made by comparison between the phase of the oscillation in center frequency and that in line width.