The most effective method for evaluating the credibility of an opinion news article from the options you've provided is:
Compare the background of the source with the subject the source is discussing.
This method allows you to assess whether the author has relevant expertise or experience related to the topic at hand. Understanding the author's background can help you gauge the reliability of their opinions and arguments.
The other options, such as determining personal agreement or disagreement, checking for consensus among the press, or categorizing the piece as editorial or op-ed, may not provide an objective measure of credibility.