@xEnjoyTheMoment The Origin model is similar the the popular Valve Steam model. If you bought just the code that vendor may retain the code in their purchase history. Once a game is activated is Steam most activation codes are not retained. But that wouldn't help you to know if the code is used or not. User accounts can not be accessed in these forums. You need to contact EA customer service - link - EA customer service about this issue.
So EA said that there's no way even for them to check whether or not a game code had been redeemed or not, which is unfortunate and honestly quite hard to believe but I guess I'll have to figure out some other way to do this. Thanks for your help and input!
If you have the game codes on hand, you can maybe find which one(s) are in use by going to your Games Library > Sims 4 > Right-click and select Game Properties. Compare the one shown to the others you have on hand.
If that doesn't help, go with what Crinrict suggested - Give one out at a time and if it doesn't work, try another.
