We can evaluate over time. If the system repeatedly predicts people you will get lung cancer in 10 years based on scans, and in 10 years we finally detect that cancer that says there must be something even if we don't know what. Finding that what then becomes important (along with figuring out how to treat lung cancer 10 years before we can currently detect it)

Of course this is all assuming there is something there. It might be there is nothing to find. I wouldn't be surprised either way.