Charles Ellicott Commentary Genesis 18:21

"I will go down now, and see whether they have done altogether according to the cry of it, which is come unto me; and if not, I will know." — Genesis 18:21 (ASV)

I will go down. —God examines before he punishes (see Note on Genesis 11:5) with the same care and personal inspection as the most conscientious earthly judge.

Altogether. —Some take this word, not as an adverb, but as a noun , and translate I will see whether they have done according to the cry of it: (in which case there shall be for them) utter destruction. But the ellipse is harsh; and inquiry, the knowing and not the punishing, is the prominent thought in the words of Jehovah. Hence too the last clause, I will know. The two angels go to Sodom to give the people a final trial. If they meet with upright treatment, then God will know that there are limits to the wickedness of its inhabitants, and it will be spared.