I’ve seen this set-up more times than I can count, and it usually follows a few well-worn paths. The simplest is a physical reveal: a rough grab during a Quidditch game leaves Harry’s sleeve pushed up, and Draco—or sometimes even Marcus Flint—sees a bruise or worse, scars. It's visual and immediate, which can work if the writer handles the aftermath well. Another common method is accidental magic or a potions mishap. Snape might brew a diagnostic potion for a class injury, and it lights up like a Christmas tree showing past fractures and malnutrition. That one often leads to a reluctant, simmering protectiveness from him, which is a dynamic a lot of readers really go for.
But the more interesting takes, to me, are the behavioral ones. A Slytherin who’s been around certain types of pure-blood families might recognize the signs not from violence, but from neglect. They notice Harry never asks for anything, never complains about being cold in the dungeons, or flinches at sudden movements in a way that’s not about battle reflexes. Theo Nott is a frequent pick for this—quiet, observant, and from a dubious home himself. He might piece it together from how Harry reacts to a casual offer of extra food from the house-elves, or the way he tenses when someone raises their voice in argument, not in anger. It’s a slower burn, but it feels more grounded in character insight than a convenient wardrobe malfunction.
The fallout is where the real story starts, though. Does the Slytherin use it as blackmail? Does it become a secret the whole house keeps, creating a bizarre, guarded alliance? I’m less convinced by the immediate 'rescue' scenarios where Draco storms off to tell his father and Lucius Malfoy suddenly becomes a child welfare advocate. The more compelling fics have the Slytherins leveraging the information strategically, maybe to unsettle Dumbledore or to bind Harry to them with a debt of silence, only for those calculated motives to slowly erode into genuine, if awkward, concern. It’s that grey-area morality that makes the premise stick for me, more than the discovery itself.
Honestly, some of the best fics I've read barely show the discovery at all—they start after it's known, and the focus is on the fragile, suspicious truce that forms in the Slytherin common room, with Harry trying to figure out their angle.