Gift card redemption
How to find out if gift card was used
How to find out if gift card was used
The balance on an iTunes Store gift card itself is either what it says on the face of the card (if present), or nothing because it has already been redeemed. The only way to check is to try redeeming the card and check your store balance. Of course that will transfer any balance on the card entirely to your Apple ID so then the card balance will be zero.
Redeem your App Store & iTunes Gift Card - Apple Support - https://support.apple.com/HT201209
Gift card redemption