It's going to be a very, very long time before we have a telescope big enough to spot little green men waving at us from the surface of another world. What we might be able to spot in the near future, though, are their farms and gardens, with a spectral technique that looks for the signatures of alien plants in polarized light.
Figuring out what's happening on the surface of an alien world tens or hundreds of light years away is a daunting task. Much of the time, astronomers can't even see the planets that they know are there, and are limited to making educated guesses about their size and composition based on how their gravity effects their parent star. Every once in a while, we get a lucky peek at the atmosphere of an exoplanet if it passes between its star and us, but that's not an alignment that happens very often.
The tricky bit about spotting planets directly is that the little bit of light that they reflect gets washed out by the light from the stars that they orbit. Astronomers, being clever, have figured out that the light reflected by a planet is different than the light from a star in one important way: it's polarized, meaning that the light waves are oriented in a measurably different way. And the degree of polarization can provide information about how much of the surface of a planet is land, how much is water, and how cloudy it is. It's even possible to detect the biosignature of chlorophyll, which could potentially provide conclusive proof of plant life on alien worlds (if alien plants use chlorophyll).
There's just one problem: astronomers say that "using this technique [to observe exoplanets] is not within reach with current technology" for the simple reason that exoplanets are really really dark and really really far away. We're probably going to have to wait for the next generation of seriously giant telescopes to come online in the next decade or so to make it happen, but by 2020 or so, we could have the first real proof that alien life exists elsewhere in the universe.