From Their God is Too Small, by Bruce Ware:

We read, “Then the LORD said, ‘Because the outcry against Sodom and Gomorrah is great and their sin is very grave, I will go down to see whether they have done altogether according to the outcry that has come to Me. And if not, I will know” (vv. 20-21, emphasis added). Open theists insist that language about God learning from what happens ought to be taken “literally” or in a “straightforward” manner. Well, consider what we would end up with from this passage if we follow this openness approach. First, we would have to deny that God is omnipresent (i.e. , everywhere present), because God says that he has to “go down to see” if what he has heard is true. This indicates, on a “straightforward” reading, that until God gets to Sodom, he cannot know whether the reports he has heard are correct. Second, we would have to deny that God knows everything about the past, for he has to confirm whether the Sodomites have done these horrible things. Evidently, then, God does not know whether what he has heard about their past actions is true, so he doesn’t know the past perfectly. Third, we would have to deny that God knows everything about the present. Because he has to go down to see, God doesn’t know right now whether the reports are true. [Ware’s italics changed to bold for formatting reasons]

Ware’s logic is sound. A straightforward reading of Genesis 18 means all those three things. Ware admits as much.