You don't have to look for oxygen etc that are the signs of earth-like lifeforms. I have no idea what prohects like this look for specifically, but any kind of

You don't have to look for oxygen etc that are the signs of earth-like lifeforms. I have no idea what prohects like this look for specifically, but any kind of thermodynamic disequilibrium that shows up in the atmospheric makeup of a planet is potentially a sign of life. Free oxygen is a very good candidate, but not necessarily the only one.