The demons look exactly the same as humans. In most manga, they have horns so you can tell them apart. If the author decides not to go with horns, I've also seen markings on the skin that look like tattoos, but are a species trait of demons. And the most rare, is that demons are the only ones that can use magic, but look exactly like humans. In this universe, I humans can clearly use magic, so I'm guessing that demons are just better or stronger at it? If you had a world where demons were the only ones able to use magic, that would be one way to tell them apart. But as things stand, how the hell is anyone able to tell a demon and human apart from each other? They seem like they are just normal humans that have been falsely labeled as demons. Hopefully there will be a later plot point to explain this. Otherwise, it's just confusing.