How do I check my gift card balance

#2 is a dead link

Redeem your App Store & iTunes Gift Card - Apple Support

excerpt at the bottom -

You can't check the remaining balance on an App Store & iTunes Gift Card. But you can check your Apple ID balance. Your Apple ID balance includes the balance from any App Store & iTunes Gift Cards that you have redeemed.

Hi, you can check your Apple Gift Card Balance several different ways,

To check your iTunes gift card balance:

1. Call 1-800-MY-APPLE.
2. Check online at www.apple.com/go/gcb/us.
3. Log in to the iTunes Store app on your Apple or PC device. Sign in to our account. Under the “Account” dropdown menu, select the option to view account balance.

I hope this helps.

Adding it to your device is the only way.