The most fundamental question when choosing a partner is a religious one. While choosing a husband, a woman should focus on the suitor’s behavior and commitment to religion. The Prophet (peace and blessings be upon him) said: “If someone with whose piety and character you are satisfied comes to you, then marry him. If you do not do so, there will be disorder in the earth and a great deal of evil.” (Reported by At-Tirmidhi) This should be the main focus of both the woman and her guardian.
It is not permissible to bring up the past of a person who regrets it, has repented, given up his sin and led a righteous life; rather, this should be covered up. The Prophet (peace and blessings be upon him) says: “Whoever covers up his Muslim brother in this world, Allah will cover him up in the Hereafter.” (Reported by Ahmad) Thus, it is unbecoming of anyone to probe into someone’s sinful past. Since Allah has covered a person’s sins, it is not anyone’s business to seek to lift Allah’s cover of confidentiality.
In many cases it is very difficult to get a true picture of a person and be sure whether he is chaste or otherwise. While taking one’s time and asking Allah for help, one can research, question, consult and ask people for advice and as a result reach a conclusion. So, it is not recommended for you to ask him about his past. A man might be sinful in the past, but after repentance he might be a true person and even better than many people who have had no tainted past.
It is worth stressing here that many of the Companions of the Prophet (peace and blessings be upon him) were people who worshipped idols, committed fornication and adultery and did evil deeds in the period of Jahiliyyah. But after embracing Islam they became good Muslims, and portrayed a strong faith in Allah to such an extent that they sacrificed their lives for Allah’s Sake and even strove to die as martyrs in His Cause.
In brief, marriage candidates are allowed to probe and ascertain the integrity and compatibility of their would-be life partners before deciding to marry them. This is generally understood from the sources and traditions and the Salaf As-Salih (righteous predecessors). However, as Muslims we are not supposed to probe into someone’s sinful past or harbor unnecessary suspicion without justification because we are required Islamically to have good faith and trust towards the believers.