Islam teaches that God is All-Knowing.

As the Qur'an is a Book of God, and is true, then this is enough to show that God is All Knowing since the Qur'an states that God is All Knowing.

But you may question the validity of the Qur'an. If you do, then you will find answers as to it's validity from knowing the Qur'an.

Have a look at this, which provides proof of the validity of the Qur'an:

http://www.harunyahya.com/miracles_of_the_quran_01.php

If you have fast internet then you can see the videos here:

http://www.harunyahya.com/m_video_de...hp?api_id=1267