How to tell when steering rack is centred?

I have been discussing the problem of my car pulling to the left with a mate who works for Mercedes and he has suggested that the steering rack is not centred when the steering wheel is - meaning that any geometry checks / adjustments are pointless. Apparently with Merc's there is a line on the steering rack / box that indicates the central position. When the rack is in this position, the steering wheel should also be centred. If people are tracking my car with the steering wheel pulled to the central position and not necessarily checking that this lines up with the steering rack, as soon as the clamp is released, the steering wheel returns to IT'S central position, meaning that the steering rack is not central when the steering wheel is. I need to know how to determine when my the steering rack is in the central position and then check the steering wheel. IF this is out, the steering wheel is going to need adjusting on its splines to match the central position of the rack. Then and only then, when I can be sure that the steering wheel and rack are central at the same time, I need to have the geometry done again. So, first of all can anyone tell me how to check the central position of the steering rack (not by counting the number of threads on the track rods)?

should have same number of exposed threads on the track rods left and right

How about counting the number of turns of the wheel from lock to lock, (after having marked the wheel ), including the fraction of a turn thats left as it wont be an even number, and then half that value, move the wheel from one full lock side, and thats your wheel centred. Mark the fractional turn with a protractor for accuracy.

Tracking correctly: Ignore steering wheel position Make sure that both track rods are of equal length (counting threads not the ideal way of doing it) by measuring from the lip the rack boot fits up against to the track rod end. Adjust the tracking, making sure that when one side is adjusted, the other is adjusted the same amount to keep the rods the same length. THEN adjust the steering wheel if necessary. The only reason for the rack/wheel to be out is if either has been removed at some point.

Spyda When I said its a lot more work to remove the steering wheel than realign the wheels, what I mean is, it is unlikely that someone aligning the wheels on the car would go to the trouble of removing the wheel to get it straight. Also, the steering wheel boss is splined, so you have to go one whole spline either way - its not a good way to get the steering wheel aligned.

Steering alignment 

The splines on the steering column / wheel are quite fine and alignment of the steering wheel should be pretty close to correct at the right spline position. There are several possible causes for the steering pulling to one side, apart from incorrect alignment. Binding / dragging brake (front or rear) or an under-inflated tyre can be the cause. I have even seen an odd size tyre fitted to one wheel. Also check tyre condition, preferably with the wheels off the car so that you can see the inside wall as well. My 740 has been pulling to the left for some time and we now suspect that the shock absorber may be faulty (all four were replaced only a couple of years ago along with the rack and all joints and bushes). This has resulted in the N/S tyre losing part of its tread on the inside.