Avraham's Recognition of Sarah's Beauty

As Avraham was on his way down to Mitzraim it says in the passuk: 

 וַיֹּאמֶר, אֶל-שָׂרַי אִשְׁתּוֹ הִנֵּה-נָא יָדַעְתִּי, כִּי אִשָּׁה יְפַת-מַרְאֶה אָת

Rashi states:
"הנה נא ידעתי" - מ"א עד עכשיו לא הכיר בה מתוך צניעות שבשניהם ועכשיו הכיר בה ע"י מעשה

How can we understand that Avraham actually did not know, because of his and Sarah's modesty, that she was a beautiful woman?

I believe that of course Avraham knew and always appreciated Sarah's beauty. It would be foolish to think otherwise. Especially in light of the Gemara's statement that it is forbidden to marry a woman sight unseen as this will lead to a man not appreciating his wife. However, Avraham always appreciated Sarah for what she meant to him as opposed to appreciating her for what other's might think. He never thought of her as a "good catch" or "trophy wife" and actually never even considered what others might think about his wife. The ultimate modesty is being a totally internal. Meaning that you are focused on yourself, bettering yourself, doing the right thing vis-a-vis your relationship with God and not letting others have any effect on your motives. Modesty and humbleness go hand in hand. I need to become the best person I can become. Others have no bearing on what I am and what I can be. Life is a contest between man and his potential not man and other's potential. The Ramban writes in his ethical letter to his son "Iggeres Haramban" to always assume that others that he interacts with are better than he. Even if it is obvious that they aren't, they have probably achieved more of their potential than he has.

Thus, for the first time in his life, on his way down to Egypt, Avraham was forced to consider what other's might think of his wife's great beauty. This because he needed to protect her (and himself) from a potential kidnapping. Here Avraham states הנה נא ידעתי, now for the first time in his married life he realizes that others might consider her beautiful too. Avraham was so great that he never stopped for even one second to think how other people would perceive his wife's beauty. That is real modesty. 

The Tanchuma adds that ע"י מעשה was that when Avraham and Sarah came to Mitzraim they came across a river and Avraham saw Sarah's reflection and saw her beauty through the reflection and that is when he realized her great beauty. What is this Midrash telling us? why reflection? 

As we stated above, the pshat in the "reflection" is that the Midrash is meaning to show us that Avraham saw Sarah from a different perspective. The reflection is referring to an outside perspective. Now Avraham was focused on how others would perceive Sarah's beauty and when he saw her reflection he exclaimed הִנֵּה-נָא יָדַעְתִּי!