"How do we know if heaven is real?"

Our human body is a like a tent, our heavenly body; a building.
2 Corinthians 5:1
"...Now we know that if the earthly tent we live in is destroyed, we have a building from God, an eternal house in heaven, not built by human hands."
v 5 "Now it is God who has made us for this very purpose and has given us the Spirit as a deposit, guaranteeing what is to come."