Propagation along an optical fiber modifies the spectrum of an ultrashort laser pulse due to interaction of the pulse with the fiber nonlinearity. These spectral modifications can be correlated with the input pulse parameters like the pulse power, pulse width, chirp, and pulse shape. We have shown earlier by numerical calculation that a small change in any of the pulse parameters is magnified in the fiber-modified spectrum due to the nonlinear nature of the interaction. We had proposed a differential spectrum monitoring technique for detecting such small changes. We now show experimentally that this technique can provide a sensitive and practical method for monitoring small changes in ultrashort pulses. Thus it can serve as a practical on-line monitor for laser stability during experiments with ultrashort pulses.