How i will know my Credit balance

My account's balance shows at the top left of the store's page on my computer's iTunes, and by logging in to view my account via the Store > View Account menu option. On my devices it shows at the bottom of, for example, the Featured tab in the App Store app under my account id (or you can view your account via Settings > iTunes & App Stores and it should show on the screen that you are taken to).

To view your account's full purchase history you can only use your computer's iTunes : if you log into your account on your computer's iTunes via the Store > View Account menu option, you should then see a Purchase History section with a 'see all' link to the right of it. Click on that and you should then see a list of your purchases. On your phone you can only see a cut-down version via http://reportaproblem.apple.com