You take the diagnostic computer and go to the EIS controller. There under actual values you will see the modules coded to the CAN B (interior CAN). You will then look at the inventory of modules reporting. This will tell you if the module is at home. If it is then you go to the Body section and pick out the OCM and communicate with it. If communications takes place you will have the oportunity to check for fault codes, actual values, and adaptations.
If it tells you every thing is OK then you have to get your wiring diagram and evaluate each circuit for what you expect from it.
If it don't talk then you need to verify that it is powered and grounded and that the CAN circuit is available. That is a little more complicated.
For more about that you can get a good description here:
http://www.mercedestechstore.com/pdfs/507%20Systems%20I/507%20HO%20CAN%20B%20(ICC)%2010-28-02.pdf