In this study, we address the topic of monitoring the drive speed of synchronous generators by time-frequency analysis of electromagnetic quantities available at the machine's outputs. In this regard, after recalling the commonly used techniques to deal with signals of scalable spectral content, we present and discuss the detection of an abrupt variation of the drive speed by suitable processing of load currents. Afterward, we propose an alternative technique based on the instantaneous frequency estimation of the stray flux. Through experimental trials conducted on a laboratory test-rig, the effectiveness of the suggested approach has been assessed under various load conditions, and its performance has been judged against those stemming from load currents processing. Promising results have been concluded in terms of generator speed monitoring by the use of a low-cost noninvasive sensor.