Probably. According to this report, a team of Japanese astronomers has tested whether a planet 40 light years from Earth–a “super-Earth” known as GJ 1214b and located in the constellation Ophiuchus, northwest of the center of our Milky Way galaxy–has an atmosphere rich in water (like our Earth) or a hydrogen-dominated atmosphere (like Jupiter or Saturn) and confirmed its water-rich nature.
But how would you perform such a test?
Easy. You perform a Rayleigh scattering test. Below is a visual explanation of how this test works:
In summary, if the sky of a given planet has a hydrogen-rich atmosphere (as shown up top), Rayleigh scattering disperses blue light from the atmosphere of the host planet. The middle panel, by contrast, shows how Rayleigh scattering is much weaker in water-rich atmospheres, while the bottom panel shows what would happen if the sky had clouds.
Image courtesy of dailymail.com.uk