chapotraphouse

IMO this is a stretch because life on another planet probably has somewhat different chemistry.

Astronomers are aware of this possibility. But because chemistry as such is pretty certain to be the same everywhere in the universe (confirmed, for instance, by stellar spectroscopy), we can study chemistry in the laboratory to understand the conditions under which chemicals are made. The more we understand chemistry, the smaller the set of unknown abiological methods of producing "biosignature" chemicals and the more certain we can be that a chemical was produced by something biological in origin. I mean yeah, we can say forever that we don't know everything (that will always be true) but there are some things that we do know pretty damn well enough.