As note for OOC metalore for those who want trainers and such, Grimjaw is fighter with battle master archetype, so he just used<em>Know Your Enemy</em>class feature, with added ability to measure 'proficiency bonus' with martial weapons (which would fit for such check). Also he has gained unique ability gain 3 information than just 2. Thanks his age and experience.<br/><br/>Edit; also added info for Ja'rok's question.