how do i get the balance of my itunes gift card?

If the card hasn't been redeemed and it doesn't have its value written on it then the only way to know its value is to redeem it onto an account.

If the card has been redeemed then its remaining amount should be on your account, the card itself is now effectively zero. 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)