Oxygen in sufficient quantities isn't too bad of an indicator, from what I understand? There are other natural processes that create it, but usually it winds up binding up with other things rather than collecting in quantities like we see on Earth. Usually.

Or so i've read. I'm not an expert. I just have google and was curious a while back. May or may not apply to gas worlds. Periods of specific activity may create false positives, but seem to recall the article saying those are usually brief, geologically speaking.

Of course, if the life isn't oxygen breathing/producing then idk.