chapotraphouse

It's ultimately going to take multiple lines of evidence. Yes, there are compounds that we think are strong indicators of life; we don't know how to explain them except for life. However, we can never rule out some other abiotic origin for that compound exists that we simply haven't discovered yet. We don't know what we don't know. Abundant life however is likely to cause all sorts of these biosignature compounds to be present in an atmosphere. If we find a planet with many of these compounds, we'll have many independent lines of evidence pointing at life being present there. That is how we are likely to finally accept that life has indeed been detected.

Another pathway that may result in the acceptance of a detection of life is us learning more about the origins of life. It's possible as we learn more about how life started on Earth, we will discover that mechanisms to get it going make it a near inevitability where the necessary conditions exist. That would make its detection much easier to accept.