Only two kind of people say with a definite conviction they had seen God or affirmatively experienced the presence of God. One, those who attains deep meditation states and the other those experienced the temporal lobe epilepsy.
In trancedental meditation, there happens a sensation of unification with the universe, or in other words completely dissolved self. On imaging the Buddhist monks who attains this state, the pre-frontal cortex is very much active, but the parietal lobe seems to be in a trance. The parietal lobe is essential for our construction of the outer three dimensional world and our interaction and spatial navigation in that three dimensional world. When this lobe switches off temporarily almost your boundary to the outer world seizes to exist. This experience reinforces the belief of God in the meditators.
Coming to those who having temporal lobe seizures, there are myriad ways of presentation. Some would experience an uncontrollable rage but blessed few could experience the insurmountable pleasure, to which no worldly pleasure could be equated , even if you imagine a person is experiencing thousand orgasms put together. Those person obviously becomes more spiritual after the attacks and to them what they experienced, as couldn't be described in mere words, is God.
If you have the intense desire to experience God, you could contact Dr. Michael Persinger, from Laurentian University in Ontario.He is having a God helmet, you know. All you have to do to see God , is to knock his doors; he would put his 'God helmet' on your head.
It would create a magnetic field which would stimulate your temporal lobe and your chances of seeing God directly correlates with your belief system.
If you are a great believer, He would appear in the form you like, right in front of you or if you are a person of beliefs and doubts mixed together, you would feels as someone like God is standing far behind you, but unfortunately if you are an atheist , all you would feel is mild ache in your limbs and some respiratory difficulties.