Thank you for your question. Once somebody asked Imam Ali (as) if he had seen God. He said "Why would I worship a Lord who I haven't seen." on the further probing of the questioner as to how that was possible the Imam said: "I did not see Him with my eyes, but I saw Him with the realities of faith." The modern world is deeply affected by logical positivism, which considers only that which can be sensed by the five senses as real. With such an outlook one can never be sure about anything metaphysical, as, by definition, the metaphysical is outside of the realm of the five senses. But so too are other phenomena that we all accept to be true, such as consciousness, and this is why logical positivism is not a tenable set of presuppositions with which to understand the world.
Rather, the metaphysical is sensed by other senses within a person and in the Islamic tradition, those senses are usually called the heart. It is the heart that senses God and this is sometimes referred to as religious experience. Through the journey of faith a person becomes more and more certain about God, through a process of incremental interactions, prayer, and appreciation of His Hand in their lives. The wisdom that is contained in His religion and the inimitability of the Qur'an. In a more advanced understanding of belief, the presuppositions that a Muslim chooses to accept mean that there is no alternative other than the existence of God and through the spiritual journey they witness the truth of their progression. God Himself guides them to the paths of certainty after which there is no doubt.
May you always be successful