Continuity mistake: How can Ron and Lavender be in a 'prominent corner' of the Common Room when in Harry Potter and the Philosopher's Stone, it says it is a circular room?
Continuity mistake: On page 186 of the American hardcover, about a quarter of the way down the page, a paragraph begins "Malfoy looked rather as he had done the time Hermione had punched him in the face." However, Hermione never punched Malfoy in the books, she slapped him in Book 3. She only punched him in the third movie.
Continuity mistake: When Severus talks to Narcissa and Bellatrix in his house, he states that he "had sixteen years of information" about Dumbledore when Voldemort returned. However, Voldemort returned in June 1995. Snape would only have been teaching at Hogwarts (and supposedly spying on Dumbledore) for not quite fourteen years at that point, as he started in 1981.
Answer: Aside from the fact that Hermione is obviously a terrible liar and her dialogue is suspiciously inquisitive for someone who often buys and sells dark artifacts, wizards can use Legilimency to determine if someone is lying.
Greg Dwyer
Legilimency is a rare talent among wizards- only Snape and Voldemort are known to have practiced it openly in the books. It is more likely that Hermoine has no idea how to act like a bad person, let alone a Death Eater.
Bellatrix was using it. She apparently taught Occlumency to Draco - Snape guesses this during a conversation with Draco.