It's been a while but isn't the first tip off the knife that the would be assassin uses?
Then, when Ned finds out about Robert's bastards and realises he was not the father of Cercei's children and that Jamie is, he puts two and two together.
Oh, I must've missed that knife-tip bit. And, was he the only one who knows about Jamie and his sister? Or, did he share that information with his wife?
I can't wait to dig into Season 2.