Go to play store, click on redeem. If the card is good, it’ll show you the balance and ask for confirmation to proceed, you can then cancel it.
If you proceed here then the card will be redeemed to your account, be careful.
If the card is already used it’ll give the “already redeemed” error.
TO CHECK YOUR GIFT CARD BALANCE, FOLLOW THE FOLLOWING STEPS:
1. CLICK ON https://play.google.com/store/apps
2. CLICK ON YOUR PROFILE.
3. CLICK ON PAYMENTS AND SUBSCRIPTIONS.
4. CLICK ON REDDEM GIFT CODE.