I know that the Quran says we should marry chaste women. I also know that men are supposed to be chaste before marriage, isn't? If that is the case how do we identify chaste men based on Quran and Hadeeth or any modern discovery?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The characteristics that are religiously required in a husband are religion and a good moral character as the Prophet said (regarding the type of man that we can consider to give our daughter to): "If a man whose religion and moral character are pleasing proposes to marry your daughter, then marry her off to him." [At-Tirmithi]

He further clarified the characteristics that are religiously required in a woman that we can consider for marriage: "Choose the religious one, you will prosper." [Al-Bukhari and Muslim] We can find out whether these characteristics are present in a given man or woman by inquiring about them.

In principle a Muslim is innocent of committing fornication (or adultery), and a Muslim woman is chaste and innocent of committing fornication (or adultery).

However, if a person lives in a country where dissoluteness, fornication and adultery are widespread, like the United Kingdom, and one fears to marry someone who is affected by AIDS or the like, then there is no harm to seek the help of modern discoveries and there is no harm for each person who seeks marriage to ask the other to undergo the necessary tests, this is of course in case of suspicion or accusation.

Allaah Knows best.