This paper presents a new Design for Testability (DFT) technique for Current-Mode Logic (CML) circuits. This new technique, with little overhead, using built-in detectors, monitors all gate output swings and flags all abnormal voltage excursions. These detectors cover classes of faults that cannot be tested by stuck-at testing methods only. Circuit simulations have shown that abnormal gate output excursions caused by the presence of a defect are common with CML. We also show that this technique works well below "at-speed" frequencies. Finally, variants of the built-in detectors with reduced area overhead are proposed.