You can recognize a really good idea by the fact that its implementation seems impossible in the first place.

Google shows that the internet often attributes this statement to Einstein, but never with a source. The English version of the quote not occur in any book in Google Books, but a forum post dated February 2010 indicates this is a translation of the German phrase "Eine wirklich gute Idee erkennt man daran, dass ihre Verwirklichung von vorne herein ausgeschlossen erscheint" which was attributed to Einstein (with no source given) on p. 350 of book Am Anfang war das Bit: die neue Bibel der modernen Physik, whose amazon.de page gives a publication date of June 2004. No examples of the English quote can be found on Usenet archives but the German version does appear in a number of posts, the earliest dated 14 May 1997.