When looking in the OpsMgr console, you cannot see which SQL Server is used for the OperationsManager and OperationsManagerDW database. There are however a few ways to find this server. I will explain them in this blogpost.
1) Install the SQL Server management pack and wait till the SQL Servers are discovered. Expand the Databases tree and type 'OperationsManager' into the look for field. Select the database and in path name you will see the server (and instance) hosting the database. (Or look at the databases immediately)
2) You can determine the SQL database by reviewing the following registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft Operations Manager\3.0\Setup\DatabaseServerName. On this location the server (and instance) hosting the database is found.
3) Go to Monitoring, Discovered Inventory and run the 'Change Target Type' task. Select 'Operations Manager Operational Database Watcher' and choose OK. You will only see one instance. Scroll to the right until you see the 'Operational Database Server Name' and 'Operational Database Name' columns.
Hope you find this as useful as I did.