Illegal Bat.

[Four Seam]

In many years of coaching at all levels and venues, I can't recall ever having seen an umpire inspect a bat when a batter comes up to bat. What I have seen, of course, is umpires inspecting the bats on the bat racks before the games. And I have thought to myself, what if, during the game, a batter brings her own bat in from her bag and goes up to bat with it, which on some teams could be almost every batter? That bat might be a banned bat, but it will probably not be looked at by the umpire. What is an umpire expected to do to enforce the rule?