Watch the direction of your questions
After choosing a suspect, notice whether every question begins with how this detail proves their involvement. Replace some of those questions with what else this detail could mean and what it would imply if another person were responsible. The goal is not artificial neutrality. It is to prevent your preferred conclusion from deciding the interpretation before you have read the source carefully.
Worked example: an unsent apology
A character's unsent apology may seem to confirm guilt. Read what the apology actually concerns. It could address a missed visit, an old argument or a misunderstanding unrelated to the crime. If the file supplies a specific subject, that subject should constrain your interpretation more strongly than the emotional tone of the word sorry.
Use a two-column check
In your own notes, place supporting evidence beside evidence that limits or complicates the theory. Include exact source references on both sides. Avoid balancing the columns by count; one strong contradiction can matter more than several vague impressions. For Somatry, this practice helps you report a theory that explains the written record rather than a character judgment assembled from whichever phrases sounded suspicious after you had already made your choice.