I wanted to ask simply, that if it is possible for people to know whether a person is right for them to marry or not through methods such as Istikhaarah why is it not possible to find out in the same way 'who' a person will actually end up marrying? Would this be classed as knowledge of the unseen?

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.

Allaah legislated Istikhaarah prayer for this nation as there is a narration of the Prophet about it. So a person whether a male or female, is permitted to pray Istikhaarah about all his matters, and about marriage as well. So if a man wants to marry a specific woman, he is permitted to pray the Istikhaarah prayer and she is as well permitted to do the same thing. To know how to pray Istikhaarah, please refer to Fatwa: 81434.

It should be noted that the reality of Istikhaarah prayer is putting the matter in Allaah's Hands to choose for you what is best and more beneficial, and then enable you to do it. This is not considered as knowing the unseen, rather the person is unable to know the unseen and he confesses to this effect. The evidence is that he puts the matter in the Hands of Allaah Who Knows the consequences of everything and Knows the unseen. So you have to implore Allaah of His Power and His knowledge. Allaah says (which means): {…that you may know that Allaah has power over all things, and that Allaah surrounds (comprehends) all things in (His) Knowledge.}[Quran 65:12]. Allaah further Says (which means): {Say [O Muhammad ]:  "None in the heavens and the Earth knows the unseen except Allaah."}[Quran 27:65].

Therefore, it should be noted that one does not know whom he is going to marry through Istikaarah or any other way because that is a matter of the unseen that only Allaah Knows.

Finally, it should be noted that it is permissible also to seek information from the people who know the person one wants to marry. So if one prays Istikhaarah and seeks consultation, the outcome would be nothing but good. It is for this reason that it is said: "The person who prays Istikhaarah will not be disappointed and the one who seeks consultation will not regret." 

Allaah Knows best.