It’s a detective’s job (or, in this case, the detective assistant’s job) to judge! It’s all about reading people and finding clues in looks and behaviour. If those judgment calls are based on years of rough experience, we’re looking at a classic detective story trope. If their assessments are wrong, even better! It could mean that the case is unusual and a challenge for the hero, or it could mean the detective is flawed or a fool (especially if it’s a comedy).