Alien civilizations do not last forever; that is science fiction. They run into problems, collapse, and cease to exist. Three of these problems have been discussed here extensively: perils of the galactic, stellar or planetary type, resource exhaustion, and idiocracy. If we ever do interstellar exploration ourselves, it would be very useful for us to know which solar systems had already had an alien civilization and what caused their disappearance.
Some sorts of perils might be recurring. Supernova can occur again and again, and the galactic bulge is no place to go looking for a new home because of them. Stellar densities are too high, and even type 1 supernova would be unpleasant neighbors. The only stellar perils that have been discussed here have been the gradual, one-time-only, evolution which provides too much heat and then too little. We don't quite understand yet about rare occurrences of stellar upsets, where blasts of stellar matter get shot into space and might hit a planet. Planetary perils like basalt floods, if they were more common than here on Earth, could be a problem for anyone seeking to settle in on a new planet. By the time we get to star travelling, our science will surely understand all of these perils, and be able to predict the likelihood of their occurrence, so even if we find a planet that had intelligent life, but it perished, we can still evaluate the planet without having to find evidence of that life.
Idiocracy having killed off a civilization is a different manner. There would be traces, and a civilization which suffers this problem doesn't exactly go away forever. Idiocracy is a genetic drift toward the loss of intelligence, but any genetic drift caused by affluence stopping evolutionary culling would do the same thing. If idiocracy or other genetic drifts occur during a time prior to the genetic grand transformation, they would not be curable by the civilization, and the population would lose the technological capability it developed to maintain a large population. Population would decline, and the slower process of evolution might start again, unless they were not sustainable at the lower population as well, for some reason. If the alien civilization had managed to strangle the natural conditions on their planet, so that not only the aliens themselves could not survive in large numbers after the loss of technology, but also the whole planet's life systems were forced into monocultures or some other cul-de-sac, then everything might fail at once, and the survival rate of the alien citizens might be too slow and they would drop below the critical mass necessary to be able to regroup and start their evolutionary climb again.
Other scenarios might include the loss of resistance to naturally occurring diseases, and when technology collapse via idiocracy or genetic drift happens, diseases might have a field day with the residual population. It is easy to state these possibilities, but hard to actually envision how the devastation, via idiocracy, loss of technological capability, food shortages, or disease could be so total and so omnipresent that the whole civilization would disappear. There may be other mechanisms which do this. Perhaps military activities could contribute to the collapse scenario and add to the likelihood that the population could go extinct. There could be some environmental contribution, if they were manipulating their weather and suddenly were unable to continue. Massive fires in maintained forests could contribute, by blackening the sky and reducing the photonic contribution to restoration of natural conditions. So the possibility exists that explorers from Earth, or a different alien solar system, could find a planet which had no alien civilization, but which had been modified by the prior presence of it.
What would matter to the new visitors is the recovery time of the planet. Recall that 'sweet spot worlds' are what alien colonists would be looking for, ones where the conditions for life would exist and all they would have to do is move in. There is a possible enantiomer problem, or something more drastic if there are DNA variants that are sufficiently efficient to cover a planet, but putting those aside, the point is that if the planet can recover in a hundred thousand years, it is likely to have done so before alien visitors arrive, given the various timescales of the galaxy, and they could simply settle down there. Maybe if the timing was lucky, they could find a few remaining artifacts.
The serious problem is extinction via scarcity. If the alien civilization progresses through the genetic grand transition, after which they have the knowledge to correct any genetic drifts, and also accumulates the knowledge and know-how to fix other problems the civilization might have, the only thing remaining which they can only postpone, not eliminate, is resource exhaustion. They would most likely do whatever is necessary to stave it off, by recycling to the maximum, and if their solar system is abundant in resources not only on their home planet, but on others, interplanetary mining and shipping of those most critical, high value to mass ones. But this postpones the day they run out; it does not eliminate it. They might reduce their population down, and stretch resource exhaustion out to a million years instead of hundreds of thousands, but the day will come. They might simply leave the planet behind and move their civilization, or go extinct in place. Either way, they leave behind a solar system bereft of resources, and until many tens or hundreds of millions of years have passed and tectonic recycling by the planet itself restores the presence of resources. Without resources, there is not much point in even visiting a new solar system. Smaller planets and satellites, if they were mined by the original aliens, do not keep much tectonic activity going and would not recycle at all. There might be the occasional deep space asteroid which impacts and adds something, but this is negligible in the whole scheme of things.
Thus, a solar system where the original inhabitants went extinct because of total, absolute resource exhaustion, or where they simply migrated to another one, same result, would be useless for explorers to go to, and more than useless for migrants to come to. This means that somehow, in order to make any of our own stellar adventures pay off with a new planet to settle, we have to somehow get our large telescopes equipped to detect the prior presence of alien civilizations, even after they no longer exist.
The shopping around we would do, or any alien civilization looking for a new home would do, involves eliminating worlds that cannot support life, because the temperatures are all wrong, or the orbits are moving and not stable, or it has the wrong atmosphere, or stellar quakes are going on all the time, or something else as bad. But this elimination process is the same for all aliens, and the same for solo planets. It doesn't help at all for finding used-up worlds and scratching them off the list. This is a harder problem than finding already occupied planets, as there is no civilization left there to turn the lights on or send mining spaceships planet to planet. There are no ruins that could be observed.
This means that understanding the origination of life is even more important. If intelligent life has tremendous obstacles in originating, so there aren't many civilizations that ever arise in the galaxy, and if furthermore, star travel has real serious problems so that most civilizations wouldn't attempt it, we might find that there are few used-up planets. However, so far it looks like any planet that can originate life can evolve intelligent life, and stellar transit isn't all that difficult for a civilization past the asymptotic technology transition. So, we have a problem.
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