How to check an Apple Gift Card balance

The best AND by far safest thing to do is to redeem it onto your Apple Account soonest.

• You can then see the remaining value at all times
• The funds are MUCH safer and will always be there until YOU spend it.

(A gift card is as vulnerable as cash; if it’s misplaced — or the code obtained by a miscreant — any value is LOST forever)

How to redeem your Apple Gift Card, App Store Card, or App Store & iTunes Gift Card - Apple Support

But to directly answer your question, see:

Check your gift card balance - Apple Support