You can check the balance remaining on all active gift cards from the Balance as on date report.
To view the report go to Reports (center level) > Marketing > select Gift Cards from the filter > select Balance as on date.
Read: Gift Card Report: Balance as on date
Note: If you need to check the balance on multiple gift cards for a single guest, then simply navigate to the guest's profile and click the Gift Cards tab.
The gift card purchased and received history appears showing the balance and other details of the gift cards. Read: Viewing Gift Cards Details.