You almost certainly can. Microbial life created the lion’s share of earth’s oxygen and along with other forms of life altered the atmosphere in many other ways. So you can study planetary atmospheres to learn about the different kinds of life that are present on a given planet, even from hundreds of light years away.
“You almost certainly can. Microbial life created the lion’s share of earth’s oxygen and along with other forms of life altered the atmosphere in many other ways.”
Correlation is not causation. Why the earth’s atmosphere has oxygen is not proof of why another planet’s atmosphere has oxygen. In fact the theories of from where earth’s atmosphere got its oxygen is not “settled science”.
http://www.latimes.com/science/sciencenow/la-sci-sn-oxygen-from-carbon-dioxide-20141001-story.html
When and if we can actually travel beyond this solar system - 100, 200, 300,+ years - there will be plenty of time to “look for alien life” outside our solar system. Right now it’s a giant guessing game with adults given plenty of expensive toys to engage in what I call “scientific crystal ball gazing”. Government funding should be no where near these soothsayers.