A monitoring procedure incorporating the fast Fourier transform (FFT) technique is presented to identify the maximum closed-loop log modulus (L-c,L-max) on-line. The proposed method addresses several potential problems present in the previous methods, which are (1) too many relay tests are required, (2) the frequency search range is confined to the third quadrant, and (3) the identified value of L-c,L-max cannot be used on-line to redesign the controller, if there is sufficient incentive to do so. Simulation results also show that the proposed method can identify L-c,L-max rather accurately as compared to the previous methods.