The only proof there can be is the sheer complexity of the geometry the circles display. That and the commonsense observation that any human being able to create installations of such power wouldn't be able to stay quiet about it for very long. And to be in so many different places at the same time - the summer solstices must be particularly hectic ! http://home.clara.net/lucypringle/photos/2003/uk03bh.html
Maybe my ambient electro mix "Gandalium - the prequel' will help to sway you but it's 'still in the mix'. I've posted some art college rock trivia at the usual place - here's the Gallic end of the agroglyph watchers. http://perso.club-internet.fr/moroaf/siteanne/page_recentes.htm The apocryphal story is that the first ones were recorded by returning WWII bombers ...