Recover iTunes username (email) Using iOS 5.1.1 
I have installed an app with an iTunes account which is not currently logged-into the 'App Store' in my iPad 2. In addition, I do not remember which username (email address) I used for that account. 
How can I find out, in my iPad, which account was used to buy that app?
If you sync with an iTunes library, hopefully you have authorised it with the account you used to purchase the app. If so, then the app would be in the “Apps” sidebar item. Click on it and press command+I. The General tab will tell you about which account purchased the item.

If you don’t sync with an iTunes library, your only option is to go to the App Store, sign in with all the accounts you know one by one, and check the purchased items for that account.