How can I check a balance on an iTunes card?

Look at the back of the card there should be a phone number or a website that you can go to and check the balance.

Hello Boliver,

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.

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