Can WE detect life on Earth from far away?
Much like the kerfluffle about phosphine on Venus, there are some compounds you just wouldn't expect from strictly geologic processes. Molecular oxygen comes to mind, but also some complicated shortish life time compounds would imply industry maybe. If we had /really/ good sensors we might even be able to spot the isotopic differences, it's been awhile since I've looked into the capacity for spectroscopy, but since the masses are different, the vibrational modes should be different, resulting in different spectra. We could conceivably spot if weird isotope distributions were in their environment, if it differed wildly enough from their suns make up you might be able to make the case that weird non-geologic nuclear reactions were taking place.
The reason the astronomy community is excited about the James Web Space Telescope is that it can do a few of these investigations.
From [0] it looks like CFC's peaked on the order of 1 ppb. Humans (perhaps) just detected phosphine at the level of ~20ppb in Venus's atmosphere through telescope spectral analysis [1], but that's on a very close planet that we've been staring at intently for 4000 years; presumably there are compounds present in smaller concentrations that we haven't seen yet. Is there a heuristic for the hypothetical minimum sensitivity relative to interstellar distances?
[0] https://www.esrl.noaa.gov/gmd/hats/about/cfc.html [1] https://www.nature.com/articles/s41550-020-1174-4
I think they would settle on the opinion that any explanation would be about as interesting as life. But not fully settle on life.