Bro. Glen
Josephus is a difficult fellow. He was, as I understand it, a bit of a turn-coat (the Massada episode). So, I would always keep that in mind when considering what he wrote. I would rank apostolic writers in the highest rank (those writings which became the New Testament canon). Secondary to those would be the men generally termed the Church Fathers (2nd,3rd century writers). Of probably equal value for the historian would be public documents (which depending on the age in question, may or may not be available). In this rank, I would also want the archelogical evidence considered. The third rank of would be secular writers contemporary to the period (e.g., Josephus, Pliny the Elder, etc.) Works other than these would probably fall into the secondary source material category. Secondary source material is of wildly varying quality, so difficult to speculate on it except on a case by case basis. Strictly academic writers, however, would likely combind the sources as those who where contemporary witnesses, plus archeological evidence as primary source material. Other works would be considered secondary sources.
When considering secondary sources or heresay evidence, you have to look at the motivations of the writer. Were they trying to preserve history for the sake of history or did they have a political/religious point of view to advance.
For example, if you are basing your whole point of view of say the Paulicans on Roman Catholic documents, then there is likely to be a slanted view of the Paulicans. A better approach, though more difficult, would be to look at the Paulicans through their own eyes, through the eyes of the Roman Catholic Church, and through non-religious eyes. The truth about the Paulicans, in this case, would likely mirror some of all three points of view, and would likely satisfy neither group who held strong views about the subject at hand.
Objectivity in history, as in most other "Soft science" is difficult to find. Some questions would come to mind when checking the objectivity of a deriviative work.
a. Did the writer look at all of the primary source material available? (Which can be a difficult task to do)
b. Did the writer consider this material (whether flawed or not)?
c. Did the writer have a strong investment in vindicating a point of view? (Was the author an internal or external writer).
d. Has the writer outlined a set of cogent questions for the inquiry? (The Who, what where, when, why and how discussed earlier).
e. Did the writer have an objective or point of view in mind before beginning the research? (Did the writer have a problem with a set of views and set out to change them, without regard for the historical evidence)?
So that, my dear brother, is a good part of the way a historical work should be evaluated. Other questions may arise on a case by case basis.
Hope it helps.
Jeff