We are getting close to being able to find out…..
By studying the atmosphere of exoplanets (those outside the solar system), we can look for gases like oxygen and methane that only co-exist if replenished by biological processes – life.
Mind you, simple life forms like microbes would alter the atmosphere in this way – it was cyanobacteria, after, that spent millennia creating the oxygen-rich atmosphere our ecosystem depends on.
If we could find traces of pollution, that would indicated more advanced life – not advanced enough to avoid polluting their own planet, but as advanced as us anyway.
The new James Webb Space Telescope should be able to detect two kinds of CFCs, provided they are present at ten times the level found on earth. This would indicated either a very irresponsible, polluting life form, or, perhaps, one that deliberately raised CFC levels in the atmosphere of a colonised planet that would otherwise be too cold for life.
This search for CFCs could also detect the remnants of extinct alien civilisations, perhaps ones that destroyed themselves by polluting their atmosphere beyond repair. Some pollutants last 50,000 years in our atmosphere and others only 10 years so if we found traces of the former but not the latter, it could indicate a civilisation that killed itself off…..or, happier thought – one that got wise in time and stopped polluting themselves into extinction….