In Spirited Away, the witch Yubaba gives Chihiro a puzzle so that she and her parents go back to human world.
In the puzzle, a herd of pigs, that may or may not contain her parents, was presented before her and she was asked to identify her parents. After a while, she guessed that her parents weren't among them.
This raised a question in my mind.
How exactly did she find it? She just paused for a while and guessed accurately. Is there any clue or plot-point I missed?