Whereas in the Solar System we can send missions which will make in situ measurements or sample returns for the analysis of the biochemical structure, in the case of exoplanets we have to do remote sensing. This remote sensing can take several forms :
There are two approaches for the detection of biological activity in the spectrum of a planet : the spectral analysis of its surface and its atmosphere.
A direct approach consists of detecting organisms on the surface of a planet thanks to their spectral properties. An example is the detection of an analog of the spectral features of vegetation, which shows a strong reflecting power above 725 nm (that's the reason why plants emit 60 times as much light in the infrared than in the green).
Instead of directly detecting the light spectrum reflected by organisms such as plants, we can try to detect non biological by-products of biochemical activity. This is the case, for instance, of gases ejected in the atmosphere such as oxygen (by-product of photosynthesis) or methane (by-product of organism decomposition). Besides, a by-product of oxygen is ozone, which we can also try to detect.