I was asked recently to find out what domain controllers Exchange 2010 deployment were pointing to. We were in the process of looking to decommission a few older 2003 domain controllers and wanted to ensure that Exchange 2010 was not pointing to them as preferred DCs.
After a bit of playing around in EMS (Exchange Management Shell) I managed to get this command working that got me out all of the information that I needed:
Get-ExchangeServer -Status | fl Name,*domain*
This should give you the list of the domain controllers that the Exchange server is running on.
Hope this helps.