Gift cards will usually have instructions listed on them for how to check the remaining balance, which may also be found under 'Terms & Conditions' as well.
However, if that is not the case, there are a few other ways that you can check the remaining balance on your gift card:
You can ask a staff member in store, they can scan the gift card for you and see what the available balance is.
You can go on to the store's website and there will be a section for Gift Cards and a place to check your balance.
You can ring the Gift Card Support number and this number is usually displayed on the store's website.
If you use a gift card for your purchase in store, the remaining available balance will be printed on the bottom of your receipt (Note: this is not the case for all stores)