That would require a spectrographic analysis. I don’t think we have the means yet.
Right now they use combined data from Hubble and Swift telescopes (which neither were designed for) to get an idea of exoplanet atmospheres. Its not very accurate and in the case of HD 189733b they can detect hydrogen because its being blown away into space.
http://www.nasa.gov/topics/universe/features/exoplanet-atmosphere.html
I know the next telescope is specifically designed to image smaller planets and should get better data on atmospheres.