Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America, states the following: “Imam Ghazali said it very nicely, ‘There does not come about in the world, seen or unseen, little or much, small or great, good or evil, advantage or disadvantage, faith or unbelief, knowledge or ignorance, success or loss, increase or diminution, obedience or rebellion, except by His will. What He wills is, and what He wills not is not. Not a glance of one who looks, or a slip of one who thinks is outside His will. He is the Creator, the Bringer back, the Doer of that which He wills.’
But this does not mean that we have no freedom to plan, choose and select. We do all these things and we are responsible for our actions.
Mawlana Mawdudi explained this point in his book Qada’ and Qadar that human freedom and divine power are not mutually contradictory. He says, “If a thing is attributed to several causes then the charge of contradiction is only applicable if it cannot have more than one cause; but if it has several causes then one can refer sometime to one cause and sometimes to another. There is no contradiction in this case.”
He explains this point by an example. Suppose it is said that water made the paper wet. Then it is said that ‘Fire made the paper wet’, then it is said, ‘Dirt made the paper wet’. This will be a contradiction, because only one of these things can make the paper wet. But if we say that the country was conquered by the King, and then it is said that it was conquered by the commander of the army, and at another occasion it is said that it was conquered by the army and yet at another time one mentions the names of individual soldiers and attributes victory to them, there will be no contradiction in this case. The conquest of a country can be attributed collectively to the head of state or the commander of the army and separately it can be attributed to the army and the soldiers.
So it is correct that the marriages happen by the will of Allah and by His decisions in the heaven, because He has created us and He has given us all the power to think, plan and decide. He controls every thing; and He knows all things even before they happen. But it is also our decision and our plan because we choose by the freedom that He has given us and we are responsible for our choices and our freedom. Allah makes decision according to His freedom, which is infinite and unlimited. We make decision according to our freedom, which is finite and limited, but it does exist. “He will not be questioned as to that which He does, but they will be questioned.” (Al-Anbiya’: 23)

Finally, doing Istikharah is recommended. It is not a “mashawarah” (consultation) with Allah. The word “Istikharah” means “seeking Allah’s guidance”. It’s a kind of Du`a (prayer) in which a person requests Allah to help him/her to reach to a good decision.”
Source: http://pakistanlink.com

In conclusion, believing that marriage is pre-destined by Allah does not mean that one should not use one’s reason or experience. Whenever there is a marriage proposal, we should try all our rational thinking and put together all the positives and negatives, evaluate every thing. In the meantime, as the Prophet, peace and blessings be upon him, told us, we should turn to Allah and seek His help. He alone knows all the hidden things and only He has the perfect knowledge of the future. He has power to make things easy for us and guide us to right decisions. This one can do by making Istikharah.