Be it with wings, horns, claws or more, there's something that marks a demon as a demon, some more than others. Whereas angels all look very similar, demons couldn't be more varied in comparison. Born of the Abyss, they are usually reviled by society.
An evil nature?
Ask a random person on the street what a demon is, and they will tell you they are vile creatures from the Abyss hell-bent on destruction... However, while this is true for a great many demons, they are not slaves to their nature. As surely as there are angels that have fallen to evil, so are there a great deal of demons that have risen to good.
Food and sustenance
Where normal mortals require food and drink, demons require neither... They instead feed on a sin, or an emotion in some rare cases. While they can feed just from the ambient presence of this sin or emotion, a demon can take more than is naturally released, withering the source.
A weakness to purity
Where angels can not handle being in the presence of impurity for long, so is the opposite true for demons. Being near someone truly pure will begin to itch, and coming into contact with pure things like holy water will physically burn them.
A demon is not doomed to staying an impure being... Just like anyone else, they can strive to rid themselves of their impurity by deeds alone. And while it is many times more difficult than for most other races, they could in theory even ascend to angelhood. Angels and demons are, after all, nearly the same race.
Demons, like angels, age differently than mortals. They become fully adult at ten years of age; seeing as they develop at twice the speed of humans before plateauing. To the point that their 'mid-twenties' seems to last hundreds of years.