The time-varying characteristics of non-linear systems responding to typical identification signals are addressed and the potential for identifying some time-varying patterns is shown. Specifically, the instantaneous frequency of system responses is analysed, and shown to characterise non-linear behaviour. The analysis is done via the Hilbert transform. Examples include a Duffing oscillator and a memoryless system of the type y = x3. The possibility of enhancing the time-frequency resolution, as compared to classic Fourier methods, is also briefly addressed.