The definition of a law as just or unjust can only come from the people whom are governed by the law in question. Although everyone should be free to decide whether or not a law is just, the only opinions that should matter are from those of the nation ruled by the law.
To formulate an answer to this question, there are several factors which must be considered on a personal level. The first of these questions is “how does the law affect me?”. If the answer is a negative one, then the individual’s opinion is already leaning to unjust. The second question to be asked is “does this law violate any other existing rights?”. If the law in question revokes or limits existing rights, then members of the nation will consider it unjust. Another question to be asked is “does this law deny or hinder any ‘inalienable’ or ‘natural’ rights?”. If the nation believes that a law violates the natural rights of humanity, then the law will be defined as unjust. The answers to these questions must all be considered before declaring a law unjust. If a law fails to meet a reasonable degree of unjustness, then it cannot be declared unjust. This “reasonable degree” is different for each individual, but is based upon the three aforementioned questions.
The best example of this comes from the abolition of slavery in the United States. The question of whether or not the law permitting slavery was just divided the nation into two halves. The North declared the law unjust, because to them, slavery was inhumane and cruel. The South, on the other hand, was influenced by the agrarian society which depended so heavily upon slave labor. To them, slavery was not unjust; it was instead a vital part of everyday life.
The decision to declare a law unjust must be made by each individual member of the nation. This decision must be made as a result of thought and reason, not passions or customs. It is based on how the law affects the individual and society as a whole. Ultimately, it is up to the popular decision of the nation to declare a law unjust.