The ColinQuinn / Black Sabbath Success Vector: You know you’re good enough to headline when you’re miserable and you destroy anyway
In the 2002 documentary Comedian, Colin Quinn explains you know you’re ready to headline "when you’re killing, and you’re up there killing, and you’re miserable. That’s how you’ll know." — http://www.imdb.com/title/tt0328962/quotes
In a November 2010 episode of VH1’s "That Metal Show," Drummer Bill Ward says that his proudest moments with Black Sabbath was when they were all sick and exhausted and picked each other off the couch and played a great show — http://www.vh1.com/video/that-metal-show/full-episodes/glenn-danzig-bill-ward/1652836/playlist.jhtml
For me, that's when I know we're ready: when we don't want to be there and we play a great show anyway.