To determine what kind of electromagnetic radiation is being detected, a physicist can:

Measuring the wavelength is the most effective method for determining the type of electromagnetic radiation being detected. Each type of electromagnetic radiation, whether it be radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, or gamma rays, is characterized by its specific wavelength range. The wavelength corresponds to the energy of the radiation, with shorter wavelengths indicating higher energy (like X-rays and gamma rays) and longer wavelengths associated with lower energy (like radio waves).

In contrast, measuring velocity would not provide distinct information about the type of radiation, as all electromagnetic radiation travels at the speed of light in a vacuum, regardless of its type. Measuring charge is not applicable, as electromagnetic radiation is neutral and does not carry an electric charge. Similarly, measuring mass would not yield useful information, as electromagnetic radiation is massless and does not exhibit properties related to mass. Therefore, the wavelength is the key characteristic that allows for the identification of different types of electromagnetic radiation.