Me too; the lack of curse may not be something they can just tell, but it's something they can certainly sense. No one ever sees Kureno, which may account for why his secret remained so long, but Shigure somehow figured it out. Whether it was because of Akito's extreme reactions to anyone other than her knowing Kureno or that there's something different about Kureno that Shigure instinctively recognizes... hm.