Answer:
Grammatically, The Question should probably be, " How is wavelength measured?". The answer depends on the particular waves being considered. Wavelength for any repeating wave is the distance between the wave where it starts to repeat. Typically wavelength is measured at the so called "Zero crossings" for electromagnetic (and many other types of) waves.
The techniques used to measure wavelength include radio detectors and counters, zero crossing detectors and radio receivers. If you can convert the wave to an electrical signal, you could also use an oscilloscope. A microphone connected to an oscilloscope for example can let you visually see, and therefore measure the wavelength of sound.
Wavelength proportional to the reciprocal of frequency, the so if you know the frequency (f) then the wavelength is c/f (c is the speed of light). You have to be careful with the units. For example a 600 Khz radio wave has a wavelength of approximately 500 meters (499.65). You have to use the speed of light to get the units right.