Can not check my credit
Need to check my balance on iTunes card
iPhone 7 Plus, iOS 11.0.3
If you mean an actual iTunes gift card as opposed to your iTunes account (View your store credit in the App Store or iTunes Store - Apple Support) then has the card been redeemed ? If it has, the card is used and its value is zero. If it hasn't been redeemed, and if it doesn't have its amount written on it, the only way to know its value is to redeem (and therefore use) the card.