Let’s assume that Venus was habitable and Earth-like up until 700 million years ago, when a catastrophic global resurfacing destroyed everything and led to a runaway greenhouse effect, which inevitably led to the Venus we see today.
Let’s say a human-like Venusian species which had technology similar to our own existed on ancient Venus just prior to the LIP period.
Would we be able to detect any traces (i.e. chemical abnormalities) or signs of their past existence on or around Venus today?
Not on the surface. (Thus a slight frame-challenge)
The chances of us landing there and speculatively excavating in the near future are negligible.
Satellites would not sustain an orbit for that long, just not likely at all.
Venus has no proper moons to leave lunar modules on for us to discover.
So.....
700 million years ago, the Earth was in full glaciation-mode (see Sturtian glaciation, AKA Snowball Earth). Protozoan life existed, nothing worth starting a conversation with.
They, like us may have explored other worlds remotely. Imagine them sending a lander with a rover onto a nice flat 2 km thick ice-pack. Aiming for the equator, if they timed it right (see this animation of continent movements through time), they might land on what is now Antarctica.
The lander/rover and all trace would get buried (a gentle blanket of snow), be compressed vertically over millions of years - and just sit there waiting for some lucky environmental-scientist to take just the right core-sample at just the right depth.
An alternative is that the crafts migrated down the glacial rivers and broke-off into an iceberg - how humans encountered that - Iceberg Towing, a bizare solution to the fresh-water crisis perhaps.
Maybe even Global-warming and the melting ice revealed the surprise to all.
It should be reasonably well preserved, excepting the plastics which would crumble and possibly be digested by microbes over that timescale. Details of their microchip-designs would be lost to thermal jiggling around - atoms shift. The basic elements should still be there in approximately the same places relative to each-other (if a bit pancaked).
How could we tell it came from Venus? It'd probably have a message with it in pictograms or other telltale "second planet from the star" type indications. Failing that we'd get the date from the materials within a few million or tens of million years (radiometric-dating - the techniques are many and varied), compare that to the particular stratum of ice, then make a "best guess" at what might work as a probable origin.
Sadly, any DNA-equivalent residue would be long-ago deranged into something unrecognizable.
Whatever was on the surface of Venus, after million of years of exposure to strong winds made of superheated sulfuric acid would have surely been eroded away. It has happened with just water and cooler air here on Earth, no reason to think it wouldn't happen on Venus.
Therefore we would hardly see signs of their presence in terms of exposed structures.
We could either find buried structures, either purposefully built, like an underground bunker, or accidentally buried, like a Venusian Pompei, or geological anomalies consequence of mining operations resulting in abnormally scarce presence of certain minerals where one would normally expect to find them.
The geological evidences might be harder to notice, because if the mining operations were happening on planet wide scale, we would see the anomalies as the norm and thus overlook them. More or less like if you took the average height of an NBA team as representative of the average height of mankind.
Probably not, quite probably never.
In this scenario, there will be fossils, both natural and technological, preserved in Venusian rocks. But the surface of Venus is far too hot to go prospecting, and there's no economic reason for a human civilisation much more advanced than ourselves to do so.
I don't think there are any unnatural gases that could survive 700 million years in the Venusian atmosphere -- Sulphur Hexafluoride, Perfluoroaklyls. (They would probably have survived 700 million years in the Martian atmosphere and we'd have spotted them by now and gotten very excited). Ditto radioactives produced in fission reactors, which have half-lives such that they'd have decayed below detectability by now.
Which leaves the possibility that ~700 million years ago, Venusians explored the solar system, and/or tried to escape the trap their planet was in. So we might find bits of Venusian technology on airless moons in the outer solar system, or in solar orbits, when we start exploring in detail. Or even on our own moon, although they would probably be buried in regolith and/or seriously eroded by micro-meteorite impacts by now.
~700 million years ago marks the geologically sudden arrival of multi-cellular life on Earth. If you want a story, maybe simple life gets spread by panspermia, so billions of years ago Venus and Earth life were somewhat similar in biochemical detail. The Venusians tried to escape Venus to Earth. They failed to preserve their technology-using species in the long term -- Venus-forming was too hard -- but they did seed Earth with multi-cellular life from Venus, and it co-evolved with native life, and so 700 million years later, here we are! And maybe there is even a fossil Venusian colony somewhere in the rocks of Earth, not yet destroyed by plate tectonics. Awaiting some geologist discovering, not a pre-Cambrian rabbit, but a pre-Cambrian not-M10 fossil nut or bolt.
The answer is similar to how we know very ancient similarly technological civilizations didn't live on Earth, radioactive isotope measurements.
Let me introduce you to the Oklo site in Africa.
When natural radioactive materials experience decay into other elements they do so at predictable rates, meaning that every naturally mined Uranium or other elements should have exactly the same ratio of isotopes.
In the 1970s Uranium fuel mined from Oklo was found to have different amounts of isotopes, indicating that nuclear fission had occurred at the site approximately 1.7 Billion years ago, most likely due to a high concentration of Uranium ore interacting with ground water to form a natural fission reaction.
If a civilization on Venus used nuclear power or nuclear reactions, ever in the history of the planet, a measurement of isotopes in the minerals collected would show it.
As an addition to the existing answers, exploring the geology of Venus might not be economically impossible for ever. We will be developing robots that can do most of their own construction and maintenance autonomously, because there is (in the long term) lots of economic incentive to develop that here. Once such technology exists (and with companies like SpaceX making access to space cheaper) it would be within the research budget of some organizations to send a self replicating robotic mining facility to Venus, and then start producing automatic surveillance rovers and aircraft to map the surface and subsurface (like with ground penetrating radar and seismology) just for scientific reasons. That mapping would then discover the fossil remains of an ancient civilization.
The best we could likely do is to figure something out by inference. If we discovered still-surviving airborne organisms in the atmosphere of Venus that was unlikely to have evolved on the planet in its current state, we would likely be able to infer that at some point there was a more habitable period in Venus's past.
If we then discovered a similar organism on Mars or one of the moons of Jupiter, it would suggest that there had been some kind of panspermian event that happened in our solar system.
But evidence might be closer than we think. If we discovered an organism living in some particularly harsh ecological niche on Earth that was related to the alien organism, but NOT to any other terrestrial life, it would indicate that whatever was responsible for the panspermian event had affected Earth as well.
So if your Venusians had by accident or intent been spreading their bacteria around the solar system, scientists might surmise that intelligent life could have put it there.
