How do you know if the law is unjust

Answer: The correct answer is : An unfair law is a code that does not agree with the moral law, the unfair law is a human law and also degrades the human personality, for example the statutes of segregation. An unfair law is not a law. If a law is unfair, a person has the right to disobey it and even the obligation not to comply with it.

The general thinking is that a law is unjust if it doesn't square with natural law. This is certainly the view that was put forth in Martin Luther King's Letter from Birmingham Jail.

So, a law that was passed that treated people differently wouldn't square with the natural law that all humans are equal. And thus, one would have the obligation to disobey that law.