A technique to measure a terahertz wave generated by spectrum tailored Fabry–Pérot lasers (FP) is assessed. A dc-biased and 25 °C temperature controlled FP is probed by a continuous wave signal, tuned at 20 nm away from its lasing modes. With a 0.02 nm resolution optical spectrum analyzer (OSA), the terahertz generated signal frequency is measured from the interval between the probe and its side-band modulations. The terahertz waves emitted by these FPs are measured at 370±5 GHz and at 1.157±0.005 THz, respectively, within a precision set by our OSA. The origin of the terahertz wave is due to passive mode-locked through intracavity four-wave-mixing processes.