I _think_ even just seeing Oxygen (in O2 form) should be fairly suggestive, because it's not stable in the presence of a whole bunch of other things.

And the CFC's would be definitive proof.

https://arxiv.org/abs/1406.3025

Yeah, but you might not be looking for those specifically.

Oxygen however. Just the fact that our atmosphere is as far out of equilibrium as it is.

I wasn’t clear at all, but I certainly agree it would be clear life was on earth from such data. I am less clear how much about our society you could meaningfully conclude from it is all, but perhaps it’d be clear from our pollution exactly what we’re manufacturing/burning

Unless the observers' planet also had ancient, non-renewable carbon deposits (which required very specific conditions) then atmospheric pollution might not even be a concept to them.

They might notice a change in our atmosphere's composition, but honestly a 200 ppm to 400 ppm change of some trace gas over the course of two hundred years could have many much simpler explanations than "intelligent life dug up all the carbon and burned it".

It has, but something fundamental to our atmosphere (even before the great oxygen holocaust) is that it has lots of potential for 'exchange'; ie, its not fully reduced. Part of our earth system depends on the reduction side (us, heterotrophs, etc..) and part depends on things being available to oxidize (co2, previously in greater quantities n2). The fact that both are observable and present would indicate that our atmosphere is being maintained at a stable(ish) state far from equilibrium.