How did Nebuchadnezzar know that the fourth man was the Son of God?

Shadrach Meshach and Abed-Nego said to Nebuchadnezzar their God would save them if they were put in the furnace - Dan 3:17. Nebuchadnezzar didn't really know that the person was God but he reasoned it was someone - angel etc - from God.

Compare these different translations:

Dan 3:25 "Then why do I see four men walking around in the fire?" he asked. "They are not tied up, and they show no sign of being hurt---and the fourth one looks like an angel." - Good News Bible

Dan 3:25 He answered and said, Lo, I see four men loose, walking in the midst of the fire, and they have no hurt; and the aspect of the fourth is like a son of the gods. - American Standard Version

Dan 3:25 He answered and said, Behold! I see four men loose, walking in the middle of the fire, and there is no harm among them. And the form of the fourth is like a son of the gods. - Literal Translation.