Abu Hurayrah reported that he was with the Prophet when a man came to him and told him that he had just married a woman from the Ansar. The Prophet asked him "Have you looked at her?" He replied "No!" He said:
"Look at her, because there is something in the Ansar's eyes."
[Saheeh Muslim, Sunan An Nasa'i and others]
Al Mughirah Bin Shubah and Anas Bin Malik reported that al Mughirah wanted to marry a woman, so Allah's Messenger said to him:
"Go look at her: it will then be more possible to have harmony between the two of you."
He went to her house. She was inside her bedroom, and he met her parents and told them. "Allah's Messenger instructed me to look at her." They remained speechless, but the young woman raised one side of her bedroom's curtain and said "I strongly reproach you! If Allah's Messenger instructed you to look at me, look. But if he did not, do not!" He looked at her, and then married her. He later mentioned, "No other woman attained her status (of love) with me, even though I have married more than seventy women!"
[Musnad Imam Ahmad, Mustradak Al Hakim and others. Sheikh Al Albani authenticated in Silsilat Al Ahadeeth As Saheehah 96]
Muhammad Bin Maslamah reported that the Prophet said:
"When it comes into a person's heart to court a woman, it is permissible for him to look at her."
[Sunan Ibn Majah, Musnad Ahmad nad others. Sheikh Al Albani authenticated in Silsilat Al Ahadeeth As Saheehah 98]
Jabir Bin Abdillah reported that he heard the Prophet say:
"When one of you courts a woman, if he could see of her as much as would convince him to marry her, he should do so."
Implementing this instruction, Jabir said:
"Afterwards, I wanted to marry a woman, so I used to hide to observe her - until I had seen that which led me to marrying her."
[Sunan Abi Dawud, Musnad Ahmad and others. Sheikh Al Albani said Hasan in Silsilat Al Ahadeeth As Saheehah 99 & Irwaa' Al Ghaleel 1791]
Based on the above narrations scholars have said it is permissible for a man to look at his wife before marriage and is in fact encouraged.
It is prohibited for a man to look intently at a woman if he is not her mahram. Looking at a woman for the purpose of marriage her is an obvious exception to this rule and must, therefore, fulfill certain conditions.
Abu Humayd as Sa'idi reported that Allah's Messenger said:
"When one of you courts a woman, it is permissable for him to look her if he only looks because he seeks to marry her - even if she does not know (that he is watching her).
[Musnad Ahmad & At Tabarani in Mujam Al Kabir. Sheikh Al Albani authenticated it in Silsilat Al Ahadeeth As Saheehah 97]
From these and other texts scholars derived the following conditions:
1. The difference between a casual look and the look of a man seeking marriage is that the latter is allowed to stare and repeat the look.
2. Looking should be for the purpose of marriage, and not for the purpose of fulfilling a desire or lust.
3. A man may only look at a woman who is available for him to marry, knowing that, should he propose, her family could possibly approve of him.
4. Looking should be without touching or Khulwah (privately meeting with her).
5. Scholars differed over what can be looked at. Some scholars limited the looking to the body parts that a woman is permitted to expose to strangers, namely, her face and hands. Others said Face, Hands and some of the hair. And others said Face, Hands and hair (without Khimar) such as Sheikh Bin Baaz.
Just as a man is allowed to look at his intended wife, a woman is allowed to look at her intended husband - with the above conditions, and noting that a mans awrah extend from his naval to his knees.