As with other controversial topics, it is necessary to take in the claims made by all sides and then to run down some of the details for yourself when a verification is necessary. Most of the time, a more thorough study of the subject will leave you with a strong conviction about which side was the closest to the truth.
But even then, we have to realize that some issues in history are not cut and dried but instead are right or wrong depending on what is being emphasized. For example, it is very difficult to be objective about Luther's stance for the simple reason that the Catholic Church of its day was in desperate need of reform (that is undeniable), but it may nevertheless be the case that the particular path taken by the reform when it occurred wasn't the right one. Or the best one. If so, that leads to the next difference of opinion as to what aspects of it were right and which were not and what standard we should use in order to decide.