how to see balance of apple gift card
How do I confirm that my Apple Gift Card has been activated?
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What do you mean by "activated"?
You can attempt to redeem the gift card, and if it redeems it's activated, valid and working. click here ➜ How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support
If you mean how to check your balance after it's been redeemed, follow the steps in the support document linked below.
click here ➜ Check your gift card balance - Apple Support