Ozone in a planet's spectrum may indicate the presence of life
Darwin will most probably look for the spectral signature of ozone, which is unlikely to exist in any quantity for any length of time in the atmosphere of a planet that is not home to life.
Note: The background image is a view of Earth seen by the Apollo 17 crew as they traveled toward the Moon. The white patch at the bottom is Antarctica. Unobscured by clouds, Africa and the Arabian Peninsula are visible at the top of this image. The large island off the coast of Africa is the Malagasy Republic.