Proof #1 - Try praying
How might we prove that God is imaginary? One way would be to find a contradiction between the definition of God and the God we experience in the real world.
What would happen if we get down on our knees and pray to God in this way:
Dear God, almighty, all-powerful, all-loving creator of the universe, we pray to you to cure every case of cancer on this planet tonight. We pray in faith, knowing you will bless us as you describe in Matthew 7:7, Matthew 17:20, Matthew 21:21, Mark 11:24, John 14:12-14, Matthew 18:19 and James 5:15-16. In Jesus' name we pray, Amen.