There are certain exceptions in Islam in which it is permissible to look at a non-mahram woman for reasons of necessity, one of which is in the case of proposing marriage. It is permissible for a suitor to look at the natural beauty of girl which will induce him to marry her. In fact it is recommended by Holy Prophet (pbuh) who said :
اذا خطب احدکم المراة فان استطاع فان ینظر منها الی ما یدعوه الی نکاحها فلیفعل (مسند احمد، ٣/٣٣٤)
“When one of you proposes to a woman for marriage, then if he is able to look at those (parts of) woman which induces one towards nikah , he should do so.” (Musnad Ahmad).
In the above hadith, the reference is towards those attractive body parts and features of a woman which induces a man towards marriage.
The natural "zeenah" of a woman are her hair, face, breasts, calves, outline of her body, curves etc which she can reveal in front of her husband, Mahram males, Muslim women, children, her slaves etc according to the Quranic verse of 24:31. These attractive body parts and features of a woman are what a suitor is interested in looking at which will induce him to choose her as his life-partner. It also also indicates that there is no sin upon the suitor if he is looking at her with lust. Allama Ibn Abidin remarks that there is nothing wrong in looking at someone one is intending to marry even with lust (shahwa).
According to Dawood Zahiri (815-883 AD), Ibn Hazm (994-1064 AD), and few others, a suitor can see the entire naked body of the woman including her two private parts (vagina and anus).
According to Imam al-Awzai (707-774 AD) and few others, a suitor can see the entire body of the woman excluding her two private parts.
According to Hanbali scholars like Ibn Qudaamah , Ibn al-Qayyim al-Jawziyyah , Ibn Hajar etc, the suitor is permitted to see what one can see of a Mahram woman i.e her hair, face, neck, chest, calves etc. The looking is definitely not limited to seeing the face and hands which a woman is allowed to reveal in front of any non-Mahrams in public. The suitor is expected to see more than that in a woman he is intending to marry. Jarir ibn ‘Abdullah said concerning his wife, “(Before marriage) I used to hide under a tree to see her.”
The suitor can look at a woman without her permission and consent. Jarir ibn ‘Abdullah said concerning his wife, “(Before marriage) I used to hide under a tree to see her.” Al-Mughira ibn Shu’bah said, I asked for a woman in marriage and Allah’s Messenger (peace be on him) asked me whether I had looked at her. When I replied that I had not, he said ‘Then look at her, for it may produce love between you.’ I went to her parents and informed them of the Prophet’s advice. They seemed to disapprove of the idea. Their daughter heard the conversation from her room and said, ‘If the Prophet (peace be on him) has told you to look at me, then look.’ I looked at her, and subsequently I married her. (Reported by Ahmad, Tirmidhi, Ibn Majah, Ibn Hibban, and Darimi).
So if a woman and her father agrees to it, then a suitor can see all or some parts of her natural zeenah. It is not very common due to cultural reasons but some families, particularly those who are religious, do agree to such demand of the suitor. Usually women of such families only show their hair and face to the suitor, and some even agree to not wear scarf which women place over their chests to hide the prominence of breasts.