How do you recognize love? How do you know kindness? How do you sense feelings? How does your heart break?
All these ideas cannot be proven but you believe them to be true because you have experienced them without seeing them.
How do we know there is a God? How can we believe without seeing Him, hearing His voice or touching His hands? How can we believe when we cannot prove that God exists?
This belief in God cannot be proven but we believe; without sight or touch. We believe because we have experienced God in some ways in our lives. We believe because we have faith.