Gift Card Balance

You redeem it, but once redeemed the entire value transfers to that account.

Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - https://support.apple.com/HT201209

Hello!

You can go to the following link to check the balance on your gift card.

https://secure1.store.apple.com/shop/giftcard/balance

Let me know if this helps!

That only works for Apple Store gift cards, not iTunes Store nor App Store gift cards