Hey there Samantha,
Welcome to planDisney and thanks so much for your question! It looks like you're just returning from your trip to the Happiest Place on Earth, so I hope you had an absolutely amazing time.
Typically, you can
check your balance on Disney Gift Cards on the Disney Gift Card website, but in order to use that feature you would need to information associated with the gift card.
When I have purchased Disney Gift Cards in the past directly from the
Disney Gift Card website, I've received an email confirmation. If you haven't already, my first suggestion would be to check your email for any confirmation messages you received that might have had that information in it.
If you don't have any luck with that, then you would need to reach out to Costco directly since they were the 3rd party vendor that you purchased your package through.
I sure hope that helps! Once you find your gift card information, remember that they can be used for practically all things Disney -
including your next visit to the Disneyland Resort - so keep the information handy when you're ready to book your next vacation!
Thanks again for your question, Samantha, and have a magical day,
Greg