While the theory that Graendal did it actually has big question marks, anyone with half a brain can come up with it. People basically just (wrongly) assume that a Forsaken must have killed Asmo, that most Forsaken are ruled out and that Graendal knows that Asmo is dead (the latter two clues come from LoC and the following books, though, contrary to RJ's words that the clues can be found before LoC).