Each game in the series identifies boss weak points differently, either by blueprints on a field journal or a PDA.
The first two games featured a field journal containing blueprints given by a wounded character (a wounded scientist in the first game, and AMS agent "G" in the second). While the first game showed only the blueprint, the sequel showed the journal flipping pages to the blueprints of the boss encountered.
The field journal does not return in the next three games, III, 4 (along with its Special expansion), and Scarlet Dawn. Due to evolving technological advancement, it was replaced by a PDA developed by the AMS, which contains a camera/scanner that photographs/scans a boss and digitally analyzes and identifies its weak point by beeps.
In The House of the Dead:Overkill, blueprints for the bosses' weakpoints are shown in the loading screens before the boss fight.