I think the basic procedure is to see what chemical signatures can be observed when the planet passes in front of its star (I believe all Kepler-discovered planets do, it's how it finds them) and then try to infer if those could have occurred through purely physical/chemical reactions or if some form of "life" had to be involved.

But, consider that there are a few places in our own solar system that "could" support life, probably with more confidence than we have in this planet so far, and yet the question is still wide open. I wouldn't expect a firm answer for many decades to come.