Ask the cashier's.
If it is a marquee cinemas card, you can check online with a creatable account at marqueecinemas.com. with this account, you can continue to check the balance in the future as well. The same goes for fandango cards, though they are not eligible at this location. You may also check via the cashier, as proposed above.
Pretty much do what Michael said. You could call them up and ask them directly. You can also Google it.
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