According to this new study, vapours emitted by plants condense into the tiny seeds that are required to form water clouds on Earth. By extrapolation, if there's complex life on an exoplanet, there's a good chance of clouds. This could make it quite difficult to directly detect the signature of exolife, because clouds tend to obscure what's going on deeper in the atmosphere or on the planet's surface.
In this case, our best bet will probably be indirect detection of biological activity by measuring the cocktail of gases above the cloud deck. In particular, a significant presence of oxygen would raise flags that something along the lines of photosynthesis could be occurring on a large scale.
Scientists recently discovered the source of naturally occurring aerosol particles in Earth’s atmosphere that play an important role in cloud formation.