I notice that you define the ones produced by unknown means as "real", and ones produced by known means as "fake". This implies that those produced by mysterious construction methods have not been produced by humans, but all it really means is that investigators don't know how some crop circles were produced. It's a big leap of faith to get from "real" crop circles to extraterrestrials, IMO.So one can't really differentiate fake from real by just looking at the aerial photographs.
---------------
Once you have evidence that some are produced with technology as yet unavailable to humans, the list of suspects other than spacefaring aliens gets pretty weird:
!. Nazis hiding out in Antarctica
2. Ghosts
3. Gods, devils, angels
4. Intelligent animals
5. A civilization that lives underwater
6: Humans from the future
7. Some well-funded covert group of humans with advanced technology they don't wish to make public
8. Artistic lightning
Whom or what am I omitting here?
But remember, there are plenty of main-sequence stars much older than the Sun within 100 light-years. Two in particular 39.2 light-years from here are approximately a billion years older and only three light-weeks apart, meaning that an alien civilization may have sent its first interstellar robotic probe a billion years ago and certainly would have also been aware of Earth that long ago, as well, just as we have become aware of exoplanets and will in the not-so-distant future will be able to detect possible M-class exoplanets.
And given the various mass UFO sightings coupled with multiple radar confirrmation, as well as statements by several astronauts and others who should know (check my quotes link above), aliens do seem to be the prime suspect in the formation of crop circles that exhibit features we can't duplicate.