Look for a setting that's not so great. If it's a world full of problems like pollution everywhere or people being really poor and controlled, it might be a dystopia. And if the main characters are young people trying to change things, that's a sign too.
You can identify the characteristics by observing the themes. Themes like loss of freedom, over - control, and a sense of impending doom are common in ya dystopian novels. The young characters in these novels are often living in a world where they have to follow strict rules that limit their creativity and individuality. There's also often a sense of a broken society. Social relationships are strained, and there's a lack of trust. The young heroes or heroines are then the ones who try to break through these barriers and bring about change.
One way to identify the characteristics is to pay attention to the power structure. In a ya dystopian novel, there's usually an unfair power structure where the young characters are at a disadvantage. For example, they might not have the right to vote or make decisions. Also, the world is often in a state of decay. Buildings might be falling apart, and there's a lack of basic resources. And the young protagonists usually have some kind of special quality or idea that makes them stand out and start the fight against the bad situation.