You can't necessarily tell if a scifi world has a "digital currency" or if everyone just happens to use credit/debit cards instead of paper money. The 1888 novel "Looking Backward 2000-1887" by Edward Bellamy is said to be the first instance of a debit card in scifi. Apparently there is no physical money in this world.
A credit corresponding to his share of the annual product of the nation is given to every citizen on the public books at the beginning of each year, and a credit card issued him with which he procures at the public storehouses, found in every community, whatever he desires whenever he desires it. This arrangement, you will see, totally obviates the necessity for business transactions of any sort between individuals and consumers. Perhaps you would like to see what our credit cards are like.
"You observe," he pursued as I was curiously examining the piece of pasteboard he gave me, "that this card is issued for a certain number of dollars. We have kept the old word, but not the substance. The term, as we use it, answers to no real thing, but merely serves as an algebraical symbol for comparing the values of products with one another. For this purpose they are all priced in dollars and cents, just as in your day. The value of what I procure on this card is checked off by the clerk, who pricks out of these tiers of squares the price of what I order."
Note that the punch-cards alluded to here was not a scifi prediction of early computers, but had already been a real-world technology for 150 years before the book was published [2,3].