This is a very common question and really starts with the pre-supposition that God exists. The first verse in the Bible is a presupposition �In the beginning God created the heavens and the earth.� No one was there to see it and the Bible does not try to prove the existence of God, it assumes it. A pre-supposition is a starting point in ones belief system that at the core says there must be certain things that are true but cannot absolutely be scientifically proven. Everyone has presuppositions, both Christians and non-Christians. In fact we all start out with the same evidence in any issue and depending on our pre-suppositions, will come to differing conclusions. That is what most people fail to understand about science. Christian scientists have the same evidence as non-Christian scientists, they do the same kind of work, they go to the same places, but because their presuppositions are different from those of non-Christians, draw different conclusions. So if you want someone prove their point, understand their presuppositions and have them prove them.
God is by nature a spirit and spirits by nature can not be seen by those in human flesh. That does not mean he does not exist, it simply means that in our finite capacity we can not point to a spirit because there is nothing physical to point to. But since that is true, how do we know that God exists? Let�s look at this in several ways.
Common sense tells us that when you see a building, one then must assume that the building had a� builder. When you look at a car, we assume that someone had to build the car, it did not get there by itself since nothing comes from nothing (which by the way is scientifically verifiable). When one sees a painting, you must assume that somewhere there was a painter and in the same way when you look at a watch we admit that there had to be a watch maker. If I stand in the middle of a highway with an 18 wheeler bearing down on me at 65 mph, and say �I don�t believe in 18 wheelers� does not mean I am not going to get hit by it.
So when one comes to our world and all the complexity it contains and we compare it to a building, or a painting or a watch, if we are honest, one is forced to consider that there must have been a creator. Christians call that creator �God.�
From a more practical side, we know God exists because of Jesus Christ. Jesus was �the God-man� or God in the flesh. He came as the second member of the Triune God head to show us what God is like and to pay the price for our sin. John 1:1 says �In the beginning was the word (Jesus) and the word was with God and the word was God.
Jesus in John 14:8 said to Philip, one of his disciples ��..If you have seen me you have seen the father�.� He is not saying they are the same person but what the Father would do Jesus was doing.
In John 10:30 Jesus said to the Jews �I and the Father are one� and they were going to stone him because according to them he being a man made himself out to be God. They knew what he was claiming.
The miracles that Jesus did, prove God exists because only God could do the miracles Jesus did, even the Jews admitted that when they asked �When Messiah comes will he do greater works than this man?� another reason we know God exists.
In addition to the above evidences are the Bible itself. If you were to come to a fork in the road, how would future people passing that way, having never met you know which way you went, or what caused you to go that way, or who you are? The only possible way for them to know you or at least something about you would be to find some record of what happened to you. The Bible is God�s record of his interaction with mankind.
History is a great record of what God did, how, when, and why he did it. In the Bible we have an accurate record (demonstrated both by Archaeology and historical record) of what God did among the peoples of the Middle East in the Old Testament which is another reason we know God exists.
So, can we see God? No! But can we know God exists? Yes! Through creation, through Jesus Christ, through the miracles he did, through science in geology, archaeology and in History. All of these things verify what the Bible teaches about God.