In Newton's book "Opticks", he proposed the particle theory of light to demonstrate the refraction phenomena. In his theory, he postulated that the tiny particles of light have mass and experience an attractive force from air to denser media (like water). Using the postulate, he derived the Snell's law and predicted the speed of light in water should be faster.
Now we know the speed of light in the vacuum is absolute and the mass of the photon is zero.
But how do we identify the fact that the photon has no mass, experimentally or theoretically? Or is there any experiment to verify the fact just like measure the speed of light?