Some people have focused particularly on clothing that's aberrant. But that in itself has little meaning, other than that the child is expressing some rebellion. Piercing parts of the body also does not mean the person is a danger to himself or others. Similarly, colors of clothing or hair do not show that a person is a threat. It can be the icing on the cake or underscore the isolation of these people, showing that they are outside the mainstream.
Social isolation can be a warning sign.
You don't have to dig deeply into someone's private inner world. Kids expose these things. They want you to know. Sometimes they have a small piece of mental health that wants you to stop them. We don't want kids to feel they must talk about feelings they are not ready to talk about.