Headscratchers / What is the Name of This Book?

• Many puzzles about knights, knaves, and normals assume that the people in question have definite knowledge of whether the others are knights, knaves, or normals. But, by the author's own admission in the solution to puzzle 106, in a situation where normals exist, neither a knight nor a knave can prove in finite time that they're not normal (since anything they could say, a normal could say, too). So, in any situation where, for example, A is a knight who says "B is a knave", how did A learn in the first place that B is indeed a knave and not a normal?