The next step is identifying biosignatures—chemicals in a planet's atmosphere that could only be there because of biological processes.
This we have done. There have been several promising signs of life based on looking for biosignatures on other worlds. It's one of the reasons we want to send missions to one of the moons of Jupiter (Europa? Titan? I forget which one specifically); because we have seen plumes of gasses that, from our current understanding, can only come from biological life processes.