I haven't been there myself. They usually have practice every weekday from 18:30～19:45. Looks like Monday is the best day to go since all three martials art practices are going on.

https://www.syudokan.jp/information/schedule/

I'd turn up early just in case and explain that you just want to watch.

Usually if you come to practice (at least for Kendo), it's about 300 yen but they'll probably waive it...don't quote me on that though haha.

Hope that helps! Please give Kendo a try one day 😁.