Animating CharacterController When Stepping Up

I am using a CharacterController (and the associated CharacterMotor scripts) to control some NPCs. I would like to know when the NPC is "walking up a step" so that I can change the animation to something appropriate.

Anyone have any suggestions for handling this? I don't see any callbacks or anything on the CharacterController for this, other than the generic OnControllerColliderHit callback.