In addressing the men who were seeking the angels, Lot pleaded with them to not do a wicked thing to the angels. Lot told them that he had two virgin daughters whom he could bring out to the men instead, if they would just leave the angels alone. See Genesis 19:1.
Genesis 19:1 says that there were two angels, not just one. Lot saw them coming to Sodom and went to meet them, bowing down before them.
It is clear from the context of Genesis 19 that the angels appeared like men, but it is difficult to determine how Lot perceived them. He bowed down before them, which could be an indication that he sensed they were angels. He also invited them to his home, offering food and lodging. The angels accepted Lot's invitation, walked to Lot's home, and ate with him.
The men of Sodom came to Lot's door and demanded that Lot bring the men out to them. They may not have known that these "men" were actually angels. When Lot spoke to the men at the door, he referred to the angels as men, saying, "...unto these men do nothing." The point of Lot's statement was that he did not want the men of Sodom to have sexual relations with the angels.