"I would think that a Dark Wizard worth his or her salt won't be showing many sure signs. The only thing I can think of that is solid 'sign' is the Dark Mark, and not too many Dark Wizards are going to roll up their sleeves and announce themselves for us. Really it's a combination of little things like a hate for Muggles or a biggotry for bloodstatus which doesn't make you necessarily dark, but if you put it all together you can start to think it would be wise to keep an eye on that person, if you catch them in the act of perfoming dark magic, or using dark artifacts, then you have something different entirely." said Jo.