There are bazillions of stars out there, and untold numbers of planets, some of which are going to harbor life. How do you go about distinguishing those few from the many barren ones? You can narrow the list by thinking about the requirement for liquid water and a reasonably stable star (it turns out those aren't so common) but you've still got a lot of objects to look at.
One possibility is that a living planet will have an obviously different landscape compared to a barren one. On earth, plant roots hold the soil against erosion by rain and wind, which then affects the speed and sediment of rivers, which can then affect the profile of mountain ranges. It is also likely that life transformed earth's atmosphere, possibly several times.
But is there something about earth that would not have occured in the absence of life? A review in Nature last January compared Earth to Mars and came up with suprisingly few definitive differences on the scale of mountains or drainage valleys. What those authors did propose is that although the range of geologic features is similar, the distribution of these features on a biotic planet might be skew detectably relative to an abiotic one.
What struck me as I read the review, though, is that our cousin planets Mars and Venus, both definitely dead at the moment, have REALLY different geology from each other, not to mention Earth. If you saw a similar object around a completely different star, you might be hard pressed to say if it was behaving "normally" (without life) or not.
I was interested, then, to pick up this paper, from the lab of MT Rosing, which proposes that photosynthetic life on earth helped create the surface energy cycle required to form the continents. The basic argument is that plate tectonics requires a lot of energy-- more than the earth's internal heating should generate. However, chlorophyll and company harvest huge amounts of the sun's light, which tilted the whole-earth energy budget in favor of tectonic movement and stable continents (basically by increasing weathering of some rocks to contribute to the tectonic churn.) But this continental drift seems to be a consequence of things which are much easier to detect, like a transformed atmosphere and tons of liquid water.
UPDATE: Molecular biologists, this problem needs you! Check out this primer (subscription, unfortunately) at Current Biology. There's lots to think about.