How can I find out the GL accounts of Profit & Loss type for which Cost Elements have not been created? 
Is there a standard report to throw these exceptions?
What is the best control to ensure that cost element is created for all expense accounts in GL? 
 

Well, the first thing that comes to mind is just download the GL Accounts, download Cost Elements and do a quick VLookup in Excel. That will tell you. That'll take you about 5 minutes or so.

or

You could try this respective transactions: 
OKB2; 
OKB3; and 
SM35 

To prevent creating of GL P&L items without creating the cost elements on 4.6 / 4.7 there is an option to allow the automatic creation of Cost Elements whenever a G/L account is created in a Company Code.  i.e. It does not create when an account is created at the Chart of Accounts Level, only when it is created in a Company Code. 

To do this; 
// Financial Accounting // General ledger Accounting // G/L Accounts // Master Records // Preparations // Edit Chart of Accounts List // Select the Chart of Accounts then; 
Integration // Controlling Integration // then I'll let you guess. 

Also, you need to specify within controlling which account ranges should be created under which Cost Element Category; 
// Controlling // Cost Element Accounting // Master Data // Cost Elements // Automatic Creation of Primary and Secondary Cost Elements // *.* 

If this was configured, you could also create a batch file now that would catch up with any cost elements not yet created. 

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