[dredmorbius]

There are a few chemical signatures that are likely to indicate life. Oxygen would be one (though there are cases in which it can exist by other means).

Spectroscopy (how signatures are detected) is more qualitative than quantitative, so it's very difficult to get a good sense of how much of something exists, as I understand. Though it's tremendously sensitive and can detect atoms or molecules across light years, and for plants, the range is ~30-300 or so from hazy recollection.

Water's almost certainly a weaker signal of life than oxygen, but it's a good sign all the same. And a strong water signal from an older but small (e.g., Earth-sized) rocky world would probably show that life does, or at least did, exist.