How do I decide, if what any given part of a text was "meant" to be propaganda, or if any misinformation found is merely a result of incomplete knowledge on the authors part?

The specific example I had in mind is a commemorative book written by a German general around 1900, dealing with the Franco Prussian war. I don't believe that it is free of propaganda, but since it deals with the course of the war, there are some parts that are accurate, as far as I can tell.

But, since a German general (probably) won't have access to French archives, there have to be some things that are the result of assumption, while others are the result of a little bit of fudging to make the vets look better.

So, how do I best separate the two? Should I just write off the whole book as trash?

(If it helps, I am trying to find out some things about how "dramatized" the war was in it's somewhat immediate aftermath. The war clearly had an important part in Germany back then (seen in the Sedantag))

(I hope all that makes sense)