Check this thread:
did your 370 get hit? file a diminished value claim!
Several of us have received diminished value checks. The Ins co., if they are legitimately working with you, will have a department or person dedicated to dealing with this. They will send an appraiser out
after the repairs are completed and he will submit a report. They will then make you an offer. There is a ton of information on the Net if you google "Diminished Value Claim".
Here's a preview:
iCan Diminished Value | Claims FAQs | Consumer Tech Support
Also, my shop, with lots of experience in this said to figure 25% of the repair cost (on average) though it will often work out to more than that depending on the circumstances. You may or may not take a Carfax hit.