There doesn't seem to be any way to detect CharacterController collision.

Run the test scene. Use WASD to move the blue capsule. If it collides with the sphere (normal collider) the log prints "hit". If it collides with the other capsule (another character controller), 9 times out of 10 it does not. The only time it does trigger the a collision method is if it is moving away from you, and you move into it from behind, it will sometimes trigger.