I've read a lot about how the spring pads on the top of the car's springs determine what size pad is currently on the car. I've also been told that you can see this with a good light and turning the front wheels all the way to the left or right respectively. Okay, now I've done this - I have a 97 E420 (W210) and as far as I can see the pad is encased in some metal cup, there's no way to see these nipples. Can someone tell me what I'm missing or not seeing?

Tolahouse
97 E420

Dont know about the W210 but I changed a set of four springs last summer on my 500SEL. There are 3 different thicknesses for my car, the sizes I cannot recall but they certainly determine the ride height of the car and the clearance of the top of the tyre to the wheel arch. What I can recall however is that I could not find a part number on these rubbers in situ but they are covered in the suspension section of the service CD's. Once the springs were out and the rubbers removed, the part number is found on the inside of the rubber ! Good luck...

You are looking at the right place. The spring pad should have little stubs sticking out slightly towards you as you're looking at it. You should be able to see it for sure if you remove the wheel.

Spring pad nubs
 

The nubs you are looking for are on the perimeter of the pad and face down. They are located in about four different locations around the perimeter so easy to see. If you put your hand in and feel the bottom side you can feel the nubs (nipples). There are usually 1,2,3 or 4 indicating the pad thickness as follows for most MBs:
1= 8mm.
2= 13mm.
3= 18mm.
4= 23mm.

Tobias MB
4 MBs