‘And Abraham went up early in the morning to the place where he had stood before Yahweh, and he looked towards Sodom and Gomorrah, and towards all the land of the plain, and saw, and lo, the smoke of the land went up as the smoke of a furnace.'
We do not know what made Abraham realise that something dreadful had happened, although he was of course half expecting it. Perhaps it was the unearthly glow in the sky, or a minor tremor which they experienced in the camp. Or perhaps he was going in order to see if his plea had been successful. Either way he rose early in the morning and made his way to the mount where he had spoken with Yahweh and there he looked down on the desolation below. What he saw was like a great furnace with smoke billowing up to the heavens.
We are not told what he thought, that is left to each imagination. The writer's concern is that we know that Abraham finally witnessed the judgment that God had warned him of, and to depict the awfulness of it.