How to check if an iTunes gift card works without using it?

Note that Apple has different kinds of gift cards.

This support article tells you how to find the balance of an Apple Gift Card or an App Store & iTunes gift card; click here --> 'Check your gift card balance' --> Check your gift card balance - Apple Support

To check the balance of your Apple Gift Card, follow this link --> https://secure.store.apple.com/shop/giftcard/balance

You can't check the remaining balance on an App Store & iTunes Gift Card. 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 the entire value, if any, to your account.

Redeem your App Store & iTunes Gift Card - How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

Then check your Apple Account balance as described in this document -->

Check your Apple Account balance - Apple Support