‘And Yahweh plagued Pharaoh and his house with great plagues, because of Sarai, Abram's wife.'
Presumably Pharaoh must have visited the particular palace where Sarai was in the harem and some dreadful illness clearly ensued there. Enquiries would then be made as to new arrivals to explain the problem, and diviners would be consulted. God may have spoken to them as he later spoke to the soothsayer Balaam. Certainly in some way they learned the truth about Sarai.
It is even possible that Abram or one of his servants might have arranged for the news to reach the ear of someone influential. He must have been devastated at what had happened and not have known what to do about it. But when news of the illness in the palace reached him he may have seen it as a God given opportunity, and acted. Alternately Sarai may have communicated the message to someone influential in the harem and spoken of what Abram's God would do in the light of the circumstances.
However to the writer the most amazing thing was that Yahweh could afflict Pharaoh. Pharaoh was a distant and fearful figure not easily approached even by Egyptians, a god, and one of whom to be afraid. But the incident demonstrates to him that the gods of Egypt are no match for Yahweh. His power is clearly all embracing. This is one of the main lessons of the account.