There are dozens of ways, but the most obvious are atmospheric gas anomalies determined by sequestration in ocean floor sediment samples, as well as telltale signs of industrial level agricultural practices in soil composition (nitrogen levels). Also, industrialization requires resource extraction on a significant scale, and we’d be finding evidence of mines in, well, the same places those resources are found now. All of this evidence would stick around for hundreds of thousands, and in some cases millions of years.
If someone was burning hydrocarbons before us, we’d absolutely know about it. It wouldn’t matter if their cities were under water.
This is an interesting point because we do find these anomalies by sequestration, there have been dozens over the last 800k years, we just call them the interglacial warming periods.
Climate change is entirely man-made except for every single time it happened before because there was no industrialization, but it totally is now tho. It's somewhat contradictory, no?
For the record, I think that they are mostly correct that there were interglacial warming periods unrelated to man, but cannot rule out some of that evidence as possibly being from an ancient semi-industrial society.
I do not believe there was some planet spanning industrial society before ours, that rivals modern civilization, so I do not believe you would be able to find evidence of it in atmospheric gases captured in the ocean. But coal burning, steam power, basic chemical development perhaps even dynamite, and material sciences, yes I believe previous civilizations have achieved that over and over and over again throughout history.
Also for your second point, as I already stated, everywhere they would have lived is under 400 feet of ocean right now so no, you wouldn't find any evidence of farming in soil samples, and any crater shaped anything anywhere on earth could have been an ancient mine, too much looney toons makes you think of old western mine shafts with wooden support beams, when real mines are just big crater shaped excavations, holes dug in the earth.
What I do believe is that anatomically modern man has been around for at least 400k years and we have evidence of even pre-modern man, not even homo sapien but a common ancestor, building wooden structures in Africa 500k years ago. our caveman ancestors were building wooden houses and defensive structures but we sat around holding our dicks for half million years?
I do not believe that we sat around rubbing our dicks for 400k years before suddenly learning how to farm. After an almost civilization ending plague wiped out 1/3 of life in Europe we went from near chattel slavery-esque sustenance farming to mapping the genome in less than 800 years. We went from "never being able to fly in a 10 million years" to landing a man on the moon in under 60 years. Humans really do not take 10k years to develop civilization, especially if there is anything from before to pass down that knowledge whether it is survivors of the previous civilization or the written word saved somewhere.
