In Musashi’s day, he tested himself by fighting life-and-death duels.  For obvious reasons, that’s not a feasible course of action for most of us.  But we can still follow his spirit by testing ourselves in life’s varied arenas.  We can go for it time and again in different fields, with different identities even.  Upon finding something that fits, dig deep until you uncover deeper truths within it, then move on to the next thing.  

In this manner, if you “know the Way broadly, you will see it in all things.”