In the Bible, there is a bit in romans that says:
For ever since the world was created, people have seen the earth and sky. Through everything God made, they can clearly see his invisible qualities—his eternal power and divine nature. So they have no excuse for not knowing God.
Even if they don't know all of the details, they should know that a God exists through what they can see in nature. If, then they are searching for this God, they will find him, because in the Bible it says "ask and it shall be given to you, Seek and you will find." How God would do this i have no idea, it is up to him. But, we can guarrantee that he would do it, because he promised it, and he doesn't go back on his promises. It says in Joshua:
You know with all your heart and soul that not one of all the good promises the LORD your God gave you has failed. Every promise has been fulfilled; not one has failed.
In India, there was a guy who became a christian through what he had seen in a dream. No missionary had ever been to this area. The guy was seeking for God, and he found God. There are probably (probably because i haven't been told of any others, but i bet that it happens more often than we think) many other instances of this.
Another theory is that since God knows each person intimately, he would know whether they would accept him if they heard about him in the Bible. However, i'm not convinced about this, as it would mean that there would be no need to tell people about God. Nevertheless, God may use a mechanism like this.
That is what I reckon anyway.