How can you check balance on a apple gift card

"You can't check the remaining balance on an App Store & iTunes Gift Card." - https://support.apple.com/HT201209 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

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