Although this technique is controversial (as in whether it works or not) some scientists think that they can use "Earthshine," or sunlight reflecting off of our homeworld to detect life on other planets.
(New Scientist Space) Earthshine--the dim glow from sunlight bouncing off the Earth, and reflected back from the Moon's surface--may aid in the search for life on other planets, say scientists. [...]
[Wesley Traub of NASA's Jet Propulsion Laboratory] and other researchers detailed how the spectrum of Earthshine reveals the presence of ozone and chlorophyll, both sure signs of biological activity on Earth. In the future, it is expected that planet hunting space telescopes will be able to resolve Earth-like planets as tiny pinpricks of light circling around their parent stars.
And though such a "pale blue dot" would not likely reveal any visual details, its spectrum might be enough to distinguish between a sterile and a living world.
This idea is probably worth exploring. Although worlds such as Mercury and Mars (if it has resources) may prove themselves to be worthwhile over time, colonizing more habitable planets would be in our long term best interests. Not only would it cost less money, but would require less energy to colonize as well.
Update: Added photo and corrected html.