George,
There is no index mark on the steering hub itself, but only on the steering shaft.
With the index mark on the shaft in the 12 o'clock position, the hub is mounted onto the splined shaft such that the steering wheel is horizontal.
There is another place to check if your steering is dead centre and that is to remove the check bolt on top of the power steering unit.
The power piston inside the unit has a little dimple that would align with the bolt hole to indicate dead centre (assuming front wheels is aligned dead straight).
There is a special bolt with a tapered tip that is used to secure the power piston in the dead centre position during installation/removal of the power steering unit.