If you want to look for a wife for someone, or a husband for a woman, then you can ask him about his wishes, such as the desired age, level of education, employment situation, country and origin and position of the family, and whether he has any preferences about her general appearance, such as skin colour, height, etc. The most important characteristic to ask about is a person’s level of commitment to Islam. This kind of useful question is fine.
But asking for details of a person’s past and wanting to know what sins they might have committed when they were ignorant about Islam – this is not right at all. Allaah covers people’s sins and loves to see them covered (i.e., not dragged out into the open). So long as a person has repented, his sins have been wiped out. Islam deletes whatever came before, so why should we ask questions that will only embarrass people? Allaah accepts people’s repentance without their having to confess or expose their sins to any other person. A number of the Sahaabah had committed adultery and murder repeatedly, or had buried infant girls alive, or stolen things, but when they entered Islam they were the best of people. No one needs to be reminded of a shameful past; it is over and done with, and Allaah is the All-Forgiving, Most Merciful. What matters when considering a person for marriage is how that person is now: is he righteous or not? Has he cut all ties with his past and his wrong deeds, or not? If he is clearly living a good and righteous life now, then it is wrong to dig up the past. If there is any fear of anything that could have future implications, such as certain diseases and so on, then there are medical tests which can give the answer and put your mind at rest.
As far as giving a detailed description of a prospective wife to someone is concerned, this can be done in writing: one of her mahram relatives (i.e. a blood relative to whom marriage is permanently forbidden) or a woman who knows her can write down a description, then a trustworthy person can deliver it to one who is seriously considering a marriage proposal. The Prophet (peace and blessings of Allaah be upon him) said: “No woman should describe another woman to her husband as if he can see her.”(Reported by al-Bukhaari, 4839)
The wisdom behind this prohibition is the fact that a husband might like the sound of what he hears, so he may divorce the one who described the other to him, or there may be temptation to do wrong.(Commentary on the above hadeeth in Fath al-Baari).
We ask Allaah to help us all to do that which He loves and will earn His pleasure. May Allaah bless our Prophet Muhammad.