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.
If the app has an update available you can download that and try entering your iTunes accounts one-by-one. Trial and error.
I do not have a sure-fire way on the iPad, but if you have a computer with iTunes installed (and perhaps even through the web), you can log into your account, check the account info and see your purchases.
Hope this helps.