Q: How can I know the presence of god ? By experience or by belief ?

A: Thanks so much for your question about the presence of God.

Belief is certainly a good place to start when thinking about God and wanting to feel his presence in our lives. When we believe, or have faith, that he is there, we can then take other steps to try and experience him more fully and frequently. And experience is always the best teacher. What the actual experience might look like may vary with each person, but I feel sure that if that experience is desired, God will make himself known.
Just to be clear, The Urantia Book has a nice section HERE on the differences between Faith and Belief. In many instances, the teachings of the book equate belief with faith. So in some regards, faith and belief can be the same.
Jesus recommends the faith of the child as an important way to open the doorway of experience with the presence of God:
155:6.12 Many of your brethren have minds which accept the theory of God while they spiritually fail to realize the presence of God. And that is just the reason why I have so often taught you that the kingdom of heaven can best be realized by acquiring the spiritual attitude of a sincere child. It is not the mental immaturity of the child that I commend to you but rather the spiritual simplicity of such an easy-believing and fully-trusting little one. It is not so important that you should know about the fact of God as that you should increasingly grow in the ability to feel the presence of God.
As students of The Urantia Book, we have the added knowledge of the indwelling presence of God as the Thought Adjuster. Speaking to the Adjuster, asking for insight, clarity, or direction has oftentimes provided me with a mental “hug” that I appreciate is from God. And it gives me a renewed sense of purpose to feel that God has heard me and responded to me.
103:5.9 It lifts man out of himself and beyond himself when he once fully realizes that there lives and strives within him something which is eternal and divine. And so it is that a living faith in the superhuman origin of our ideals validates our belief that we are the sons of God and makes real our altruistic convictions, the feelings of the brotherhood of man.
And there are other ways to increase our awareness and experience of God: being born of the spirit, meditation, and worship, for example.
When once we believe in God – have faith in his presence – we can even attempt to contact him. We can get to know God personally. And this is an achievable experience, according to the teachings of The Urantia Book. God can be known, person-to-person. And this kind of experience can be life-changing.
And here we come across one of the best ways to know and learn to experience the presence of God – through knowing more about Jesus.
169:4.4 Jesus well knew that God can be known only by the realities of experience; never can he be understood by the mere teaching of the mind. Jesus taught his apostles that, while they never could fully understand God, they could most certainly know him, even as they had known the Son of Man. You can know God, not by understanding what Jesus said, but by knowing what Jesus was. Jesus was a revelation of God.
1:6.8 God is spirit—spirit personality; man is also a spirit—potential spirit personality. Jesus of Nazareth attained the full realization of this potential of spirit personality in human experience; therefore his life of achieving the Father’s will becomes man’s most real and ideal revelation of the personality of God. Even though the personality of the Universal Father can be grasped only in actual religious experience, in Jesus’ earth life we are inspired by the perfect demonstration of such a realization and revelation of the personality of God in a truly human experience.
And so, we learn here that studying the life of Jesus is another way to help us experience God in our own lives, as Jesus did. We need only to follow his example to know how to communicate with God. Jesus did it through meditation, and just “having a little talk” with God.
Again, Jesus teaches:
155:6.3 “Your religion shall change from the mere intellectual belief in traditional authority to the actual experience of that living faith which is able to grasp the reality of God and all that relates to the divine spirit of the Father. The religion of the mind ties you hopelessly to the past; the religion of the spirit consists in progressive revelation and ever beckons you on toward higher and holier achievements in spiritual ideals and eternal realities.”
The Urantia Book teaches a great deal about personal spiritual experience. You might enjoy reading through our topical study that condenses some of those teachings in one place  HERE

And thank you so much for writing with this important question about the spiritual life.