King Hezekiah's recovery from a fatal illness involves a dramatic sequence of divine promises, medical treatments, and miraculous proofs. Yet, the placement of the king's request for a sign at the very end of the narrative creates a chronological puzzle, since God had already provided a miraculous sign earlier in the story. To resolve this timeline issue, [רד"ק, אבן עזרא] explain that the king's request is recorded retroactively. He had actually asked for proof of his healing earlier, and the miracle detailed previously was the direct response to that past request.
Looking at the parallel account in the Book of Kings, [מלבי"ם] points out that the events unfold in a much more logical order: the prophet instructs the king to apply a medical poultice, the king asks for a sign, and only then is the miracle performed. This contrast raises questions about why the sequence is reversed in this specific account, as well as why such an earth-shattering miracle was necessary for one individual's private healing.
While these approaches treat the king's words as a literal question, another perspective views them not as an inquiry, but as an exclamation of awe and deep gratitude. In this light, the phrasing emphasizes the immense greatness and importance of the event [מצודת ציון]. The king is expressing profound wonder and joy, declaring how beautiful and wonderful the divine sign is. It serves as a joyful confirmation that he is truly healed and can once again visit the House of God [רש"י, מצודת דוד].
A final approach ties the king's words directly to his physical recovery process. After receiving the prophet's medical advice to treat his wounds, the king wonders how he will know when he is completely cured. The answer lies in his upcoming journey. Once he feels strong enough to rise from his sickbed and travel to the House of God to offer his thanks, that physical journey itself will be the ultimate proof. The sheer ability to reach the proper place of thanksgiving will serve as the true sign of his complete healing [ביאור שטיינזלץ].