The question asks what Mr. Yamada should check before applying. The fee for the afternoon course (800 yen), the need to bring lunch, and the eligibility of his son (elementary school student) are all clearly stated in the flyer. However, the capacity is limited to 'the first 20 pairs for each course' (各コース先着20組). Therefore, the most crucial thing to confirm before applying is whether the course is already full or if applications are still being accepted. The other options are information that can be found just by reading the flyer.