Genesis 29:25

And it was the time of the morning and he saw her, and behold, she was Leah, whom all the night he had thought to be Rahel; because Rahel had delivered to her all the things with which Jakob had presented her. But when he saw this, he said to Laban, what is this that thou hast done to me? Was it not for Rahel that I served with thee? Why hast thou deceived me?

והנה היא לאה, whenever something had not been known or recognised previously the word הנה alerts the reader to this fact. (compare Genesis 41,7 והנה חלום, “it was a dream.”) Until Pharaoh awoke he had taken what he saw in the dream as being real.

ויהי בבוקר והנה היא לאה, for he had not recognised her until morning as we explained. The aggadic explanation mentioned by Rashi that Rachel had given her sister Leah the codeword which she and Yaakov had agreed should serve to preclude such a swap is well known.

ויהי בבוקר, “when it had become morning, etc.” during the entire night Leah had pretended to be Rachel, using three different items to identify herself as her sister, which Rachel had received from Yaakov, one had to do with the laws of menstruation, the second one with the laws of challah, and the third one with the lighting of candles on Friday night. We are told the following in B’reshit Rabbah 70,19: all night long people were singing what sounded like the praises of Leah, sort of hinting to Yaakov that he was being swindled. Yaakov, who had no reason to fear that the town’s people wished him ill, did not suspect being tricked when Leah always answered when he addressed her as Rachel. In the morning when he found out that he had been tricked by Leah, and he complained to her, she responded that she had learned from him to do this, as when Yitzchok had asked him if he was his firstborn son Esau, he had answered in the affirmative. This is when Yaakov began to hate her as is reported in verse 31, where G–d is reported as taking note of that. Yitzchok himself had told Esau that his brother had tricked him.