Harry was suddenly more aware of the package under his cloak. That was the proof, he assumed, since Snape hadn't known why he'd need the Snitch. It wouldn't make any sense for a Snitch to prove anything about this, anyway.
He swallowed, and then nodded. It was too surreal for him to honestly say that he believed Snape, but on some level, he did. "I will," he said. That much he could definitely promise. "And I'll do - what needs to be done. For him to be defeated."