Here's why:
* Their behavior: The King and Duke are constantly scheming, lying, and manipulating others. Huck, with his frontier wisdom, sees through their charades.
* Their inconsistencies: Their stories change and contradict each other frequently, revealing their lack of truthfulness.
* Huck's skepticism: Huck is a pragmatic character, not easily swayed by fancy talk. He relies on his own observations and intuition.
* His internal dialogue: Throughout the novel, Huck expresses his disbelief in the King and Duke's claims in his internal monologue.
While Huck initially allows them to stay with him and even participates in their scams, his skepticism never goes away. He understands that the King and Duke are dangerous and untrustworthy. This understanding drives his actions and contributes to his growing moral compass.