This paper describes the use of a spectrally-based "nonlinearity indicator "to complement ordinary spectral analysis of jet noise propagation data. The indicator, which involves the cross spectrum between the temporal acoustic pressure and the square of the acoustic pressure, stems directly from ensemble averaging the generalized Burgers equation. The indicator is applied to unheated model-scale jet noise from subsonic and supersonic nozzles. The results demonstrate how the indicator can be used to interpret the evolution of power spectra in the transition from the geometric near to far field. Geometric near-field and nonlinear effects can be distinguished from one another, thus lending additional physical insight into the propagation.