The parsha starts off with the story of the three angels who come to Avraham appearing as men. They eat, drink and make merry and give over the information they need to give over. In 18:9 the angels ask Avraham "Where is Sarah your wife?"
If Avraham did not know they were angels, how was he not curious how they knew he had a wife and her name was Sarah that they should ask about her? And if he did know they were angels, why did he bother with the trouble of making lunch for them, slaughtering a bull and baking bread, etc, knowing they would not eat it because they are angels?