Daniel 3:25 (KJV) He answered and said, Lo, I see four men loose, walking in the midst of the fire, and they have no hurt; and the form of the fourth is like the Son of God.

This is Ellen White's commentary on Daniel 3:25
Kings and Prophets
Page 509
Why the king recognized Jesus

How did the heathens king [Nebuchadnezzar]know what the son of God was like? The Hebrew captives filling positions of trust in Babylon had in life and character represented before him the truth.... Plainly and simply they had presented the principle of righteousness, thus teaching those around them of the God whom they worshiped. They had told of Christ, the rmRedeemer to come; and in the form of the fourth in the midst of the fire the king recognized the Song of God.