Question

How do you measure the wavelength of a light?

Answer 1

For a rough measurement, you could set up essentially any experiment whose results depend on the wavelength. For example, reflect a beam off of a diffraction grating, and measure the angle of reflection. This essentially means building a type of spectrometer.
An instrument that measures the wavelength of a near-monochromatic beam very precisely is called a wavemeter. Wavemeters can be built on several different principles, but common ones include the Michelson inteferometer and the Fizeau interferometer.