We’ve done this before. In these sorts of time frames you really are reduced to what gets fossilised, and that in turn depends on the physical and temporal extent of your civilisation. 50 million years ago forests covered large areas of the earth, but you don’t see vast amounts of evidence for such forests today despite their mass and vast extent. Only in some very tiny areas you can find fossils of forests from 50 million years ago.
People tend to think that there are fossils everywhere, and thus everywhere has fossils from 50 million years ago. That isn’t accurate. There are fossils in lots of places, but they come from all different ages. Trying to find fossils from a single ecosystems from a single period is literally the lifetimes work of many palaeontolgists. If you want to find fossils of the first flowering plants, for example, you might need need to find fossils of freshwater ecosystems from a period of 5 million years, starting 150 million years ago. And people literally spend their lives looking.
What we are discussing here is very similar. Rural areas just won’t fossilise. Even the most intensively worked farmland will be unrecognisable within a few thousand years at most. Someone in the earlier thread suggested we would get petrified cities like we get petrified forest, which is ridiculously improbable. Forests covered trillions of hectares for hundreds of millions of years, with large areas growing on swampy ground ideal for flooding an in situ fossilisation. An still we have a few of hectares of petrified forest in the whole world. Cities might have covered a few hundred thousand hectares for maybe a hundred millenia, with a few thousand hectares on suitable swamps and floodplains. The odds of an actual petrifeid city is basically nil. You really need to be looking for sediments where garbage from cities is being washed.
If there was a single regional civilisation, akin to Mespotamia 10, 000 years ago, and it lasted 100, 000 years, you would never find it. The odds of any evidence even being fossilised is staggeringly remote. The odds of the fossils lasting 50 million years is even more remote. The chances of the fossil bed being revealed by erosion are miniscule, and the odds of someone who knew what they were looking at finding the exact portion of the fossil bed that contains the incriminating fossils is about as close to zero as makes no difference.
If you had a truly world spanning civilisation as we have today you might have better odds. We are producing so much garbage now that we have probably left some evidence of our existence in most fossil beds. But the evidence is still dispersed. There might be one electric drill or equivalent in ever hundred square metres of ocean sediment. That’s still not a readily detectable signature but it might show up.
Then we come to the issue of attribution. We find odd fossils all the time today. We don’t attribute them to technology, we simply file them away as “unclassified” and they languish in museum drawers for decades. Occasionally someone knew has a stab at what they are. Even if someone did find a fossilised circuit board from this civilisation, it wouldn’t be attributed to technology unless it was clearly identical to their own technology, which is unlikely, as noted in that other thread. Instead it would be published with guesses about it being an odd mineral formation or the tunnel system of some burrowing worm and then it would be put in the museum drawer.
The same attribution problem applies to other evidence such as radioactivity, species distributions and similar. We already find areas of unusually high or low radioactivity, shocked quartz, odd species distributions and so forth. We invent rational explanations for them without ever invoking ancient civilisations.
To convince scientists that you had a real civilisation you need something that is clearly built by intelligence. That isn’t that easy to do. Most furniture could be ascribed to unknown organisms. Broken fragments of things like circuit boards the same. You really need an intact piece of complex equipment like a car or DVD player.
Unless a civilisation lasted along time over the whole world, I wouldn’t put money on finding any convincing evidence that it existed.