To see how many Bitcoins are on the Bitcoin Gift Card, you need to access the Blockchain.
All Bitcoin transactions are recorded here. Completely anonymous, of course. No one can guess that your public key belongs to you
Visit the Blockchain on the following page:
https://www.blockchain.com/de/explore
Just enter your public key at the top of the search bar. You will now see how many Bitcoins are located at this address. The public key is your Bitcoin address. Depending on the design, the card will say “Public Key” or “Bitcoin Address” on it.