A telltale sign is something that reveals or gives away information that might otherwise be hidden. When detectives notice telltale clues at a crime scene, those details point clearly toward what happened. When a teacher spots telltale signs that students copied each other's homework (identical unusual mistakes, same creative examples), those matching errors reveal the truth.

The word works because these signs “tell the tale” of what really occurred. Telltale tracks in fresh snow show exactly where the rabbit went. Telltale chocolate smudges around your little brother's mouth reveal who ate the cookies, even if he denies it!