detect optical transceiver transmit wavelength
While Marcus Miller's answer is entirely correct, if you are driven to answer your question the instrument you want is an OSA (Optical Spectrum Analyzer). These are extremely high-resolution spectrometers, usually employing FTIR spectroscopy. Wavelength accuracies of about 1 ppm are available, but the best units will cost in the 5 figures. eBay is a good place to start looking, although you can start with companies such as HP (now Agilent Oops, now Keysight), Advantest, and Yokogawa. Or, if you're feeling really masochistic, Thorlabs has a very nice selection.