Draco apparated to the muggle world and discovered two Death Eaters duelling on an empty street. A muggle girl was watching from around the corner. He rushed to her side, and turned her sharply to face him. He silenced her with a spell and with a flick of his wand, he called for the dementors which would collect the duelling filth. The girl gasped. She saw them too. And so he knew, that she couldn’t be a muggle. Muggles can’t see the dementors. Maybe a late bloomer?