Many social phenomenon are most easily described in this way. Generally, the easiest way to find out who is is a member of a subcommunity is to ask other members of that subcommunities.

And as Migeru said, its a combination of that influence among the rest of the mainstream media - and thus general public discourse, which they catalyse - and audience numbers - some influential magazines are more "insider media", being read by those that make the news, but not really bringing these news to the public.

Un roi sans divertissement est un homme plein de misères

Wikipedia: I know it when I see it

Justice Potter Stewart used the phrase in his concurring opinion in Jacobellis v. Ohio 378 U.S. 184 (1964). He wrote:

"I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that." (emphasis added)