Recently developed Active Quenching Circuits (AQCs) with fast-gating capabilities allow us to control a single photon avalanche diode with gate windows in the nanosecond and sub-nanosecond range, thus paving the way to advanced applications, especially in the field of time-correlated single photon counting. In this scenario, an accurate measurement of the time needed by the AQC to turn-on the detector is of utmost importance. Indeed, it permits us to evaluate the impact of the system in specific applications and provides a tool to designers to understand AQC limitations and to enhance its performance. Here we propose a simple non-invasive technique to accurately measure the time needed by a gated system to turn on the detector. The effectiveness of the measure has been proved on a gated system, and results have been compared to those obtained starting from the distribution of recorded photons under constant illumination, which is a widely used approach in the literature. The great advantage of the proposed approach is that it avoids typical artifacts that affect other kinds of measurements.