Balance on iTunes card

Note that Apple has different kinds of gift cards.

This support article tells you how to find the balance of an Apple Gift Card; click here --> 'Check your gift card balance' --> https://support.apple.com/HT212864

You can go to the following link to check the balance on an Apple Gift Card (but not an App Store & Tunes) gift card. --> https://secure1.store.apple.com/shop/giftcard/balance

You can't check the remaining balance on an App Store & iTunes Gift Card. The balance on an iTunes or App Store gift card itself is either what it says on the face of the card, or nothing because it has already been redeemed. The only way to check is to try redeeming the card which of course will transfer the entire value, if any, to your account.

Redeem your App Store & iTunes Gift Card - https://support.apple.com/HT201209

Then check your balance as described in this document -->

View your Apple ID [not the balance on a gift card itself] balance - https://support.apple.com/HT202359