To begin with, a journal is a problematic source of information because it is usually the impressions of the person who writes it, and therefore they are very subjective. In the case raised, the journal is even more problematic because it is written by a citien of East Berlin in a period marked by political ideology.
To check if it is a reliable source, I would begin by trying to find out as much information as possible about the person who writes it to get an idea of his or her personal and ideological background since it will set the tone of the narration: ideology, in which area of East Berlin he or she lived, where he or she worked.
Secondly, I would look for other contrasted sources that mention this journal to know if it has already been taken into account and studied or if it has been discredited.
Then, in order to contrast the information present in the journal, I would look for specific places, dates and events reported by the writer and compare them to newspapers, periodical reports from that time or other bibliographical sources (public libraries or databases) to assess the author's rigour.
In no case would I study the journal as an isolated source without comparing it to others to interpret the information available.
