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 all the balance, if any, to your account.

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

You can go to the following link to check the balance on your Apple Store (but not iTunes Store) gift card. - https://secure1.store.apple.com/shop/giftcard/balance

Limnos wrote:

You can go to the following link to check the balance on your Apple Store (but not iTunes Store) gift card. - https://secure1.store.apple.com/shop/giftcard/balance

Thanks...I was not aware of that link!

You have to redeem your Gift Card to your Apple ID balance. Please refer to Redeem your App Store & iTunes Gift Card - Apple Support.