I’m afraid I’m not sure what “EXISTING” means, but any valid credit card, whether saved in Square or not, will always produce a card nonce, also known as a token. Pretty sure they’ve moved the lingo to refer to card “token” as it seems more in-line with current language.
Before you use the card to process a transaction, or save it, you only have access to the nonce, so you can’t see card details at that point.
Once you’ve processed the card, you should get back an object which has all the details, including card brand (Visa etc), first 6, last 4, expiry date, and so on.
Is that what you needed to know?