There are multiple ways to check the balance on a gift card.
From the Appointment Book:
Click any empty block on the Appointment Book
Click Prepaid/Gift Card Details from the context menu
Enter the gift card number in the pop-up box and click Show.
To learn about the details displayed in the window, read: Read: View Gift Cards Details from the Appointment Book
From the Guest Profile: Navigate to the guest profile and click the Gift Card tab.
To learn about the details displayed in the window, read: Read: View Gift Card Purchased and Received History.
Existing guests can log in to their online account and check the balance of the gift card themselves. Guests can follow the steps in View the Gift Card Balance Online.
Note: If the gift card was purchased online, without the guest signing in (used the 'continue as guest' option), then you can check the balance only from the Gift Card Details report. Read: How do I track gift cards purchased online using the ‘Continue as guest’ option?.