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?
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.
Mechanical logic gate is just a fancy lever assembly. Hardly somebody name them so, but they was used a lot, say, door lock could be named mechanical computer too.
Doesn't look very precisely cut and definitely not 1000+ tons. Some looks like somebody just select suitable stones to minimise surface grinding to fit them together. May be not very effective way to build the walls or foundation, but definitely in human abilities of that time.
I wouldn't even be surprised with using concrete thousands years ago.
If we don't know exact technology used, that does not mean that it is something out-of-time.
Better example would be Antikythera device, since it is a relatively complex mechanical thing appeared without any other, may be simplier things of same design and technology. It definitely could be made using 100 BC technology level and instruments, but there are no any known predecessors or something similar at all. It is like a i8086 microprocessor made right after discovery of semiconductors, without steps like i8080, i4004, simple logic ICs, hybrid ICs, discovery of transistor, discovery of diodes and all that stuff.