When Voldemort did the Killing Curse on Harry in The Deathly Hallows Part 2 and he asked Narcissa to see if Harry was dead, and she asked if Draco was dead - why did she think Harry knew if he was alive or not? It's not like they're brothers.
She wasn't sure Harry would know, but he was more likely to know than anyone else there.
He'd been on the inside of Hogwarts, with the defenders, and she knew Draco was also inside. Stuck in the Death Eater camp, she had no way of knowing what was happening to Draco or even of getting in contact with anyone who knew - until Harry came along. He was the only person in that clearing who might have a chance of knowing whether Draco was alive or dead, so she took that chance and asked him.