How do I check if a PSN voucher code was already redeemed on my account?

On PS5 or PS4: go to Settings → Account Management → Account Information → Transaction History. Wallet top-ups appear as "Wallet Funding" and game or subscription codes appear as the product name. On the web: sign into account.sonyentertainmentnetwork.com and check Transaction History. Also check your Library for any games that may have been added, and check whether a PS Plus subscription is now active on your account.