If you want to find alien technology, you need to find an example of complex techology appeared from nowhere, without any prior findings.
Complex technology is developed step-by-step, adding small bits of inventions to previously known ones. It is easy to track them to the beginning.
Check everything you know with that criteria, and if you can't find that prior steps, there is definitely something very interesting.
Example:
There are rumors, that microchips is an alien technology that was stolen from alien ship. Let's check it.
Semiconductors was discovered long ago, when people study electric properties of known substances, and it was definitely not an alien influence. Then, the properties of junction of slightly different semiconductors was discovered. Again, simple step that gives us semiconductor diodes and LEDs. It was only question of time and curiosity, when the properties of triple junction of semiconductors will be studied and transistor will be discovered. Transistors produced by putting a crystal in metal case, and obviously people began to put another elements like resistors and capacitors, that usually take a space on PCB, along with transistor crystal into one case. That was first hybrid ICs. Also simple and obvious step. Hybrid ICs become more complex, including dozens of transistors and other componets, taking all space in the case, and next step was to put all elements on a single crystal. That is microchips we know today.
Here we track step-by-step technology development with small rational steps, that humans definitely was able to do. So this is a normal human techology, and there was no any need for alien "help".
However, there could be something where we could not find the history of invention. Say, some medicines looks suspicious - once some scientists just synthesize some very complex chemical compound, and think that it should treat some disease. They check it, and it really works. Taking in account, that it is still not very clear how human body works, so you can't somehow calculate necessary compound, and there was no any history of studing similar or simplier chemical compounds to treat that disease, the probability that it is just a coincidence is very low. It is also hard to imagine that scientists continuously create random substances and try if they work. It would take a millions of man-years to find something working. So, there should be some hidden background knowledge or an external influence.
Could you find more suspicious examples of complex technologies that appeared from nowhere?
May be. Or may be not. And we don't know the technology used.
I could imagine some alien builders, who occasionally help earthlings during occasional fly-by, leaving us with a mistery, but they didn't share a technology with us.
I also could imagine the simple technologies absolutely possible for that time, say, wet sandblasting to precisely cut the stones and kind of linear bearing using wooden balls to move them. Yes, that will take a lot of time and people, but what ruler cared about that in the past?
Really, the question is not about some out-of-place or out-of-time artifacts, where everybody could think out multiple explanations, but about a known technology. Technology is less noticeable than strange artifacts and its history attracts less attention, so there are less probability that somebody will spend a lot of effort to fake traces of step-by-step development, and at the same time it is much harder to fake that traces to make technology look authentic.
If you have 5000 slaves, moving cutting and placing 1000+ ton stones becomes just a matter of time. You could cut ultra hard rock just pouring a water with sand from some vessel put high enough to get a decent speed of running water. And it will make cuts of any depth and very precise. But we don't know, if it was done that way, or not. So, sole existence of artifact is not a proof.
As for linearity - of course there could be important breakouts, but they are atomic, single-step things. As in example of microchip, every step was a huge breakout, sometimes even occasional, but none of them could be compared with the hypotetical situation, if microchips appeared after discovery of semiconductors without any intermediate steps. That kind of breakout will definitely be very suspicious.
Menwhile I think capacitors in a form of leyden jar was invented just by similarity - a scientist literally tried to pour electricity into a jar. But that was a consequence of studying electricity - just another step in a long way. And there was no need in something better that layden jar at the time. As soon as that need appeared, people quickly figure out how to roll a foil and insulator to make a compact multilayer layden jar. And that was a capacitor that still widely used everywhere. Of course, we use thinner foil and polymer insulator much better that paper user at the beginning, but it is literally the same capacitor. Next steps of inventions was using electrolyte and layer of oxide as insulator to make electrolytic capacitors and using thin film tech to create ceraminc capacitors. Still there is sequential and quite rational steps.
I hope you get the idea - look for some complex tech that hardly could be an occasional one-shot invention and definitely need multiple steps to appear.
Do you have some example?
You mean something like Stonehenge? I hardly could name it as something impossible. I thought more about something like Baalbek.