Which Key Code Did I Use?

[Shamanos]

I purchased a retail copy of a game and added it to my Steam account. I never installed/played it, so, at a later date, forgetting that I even owned it, I purchased the exact same game again. When I tried to add it to my account, of course, Steam informed me that I already had it. Disgusted with myself for being such an idiot, and realizing that I'd be unable to return it now that I'd opened it, I tossed it on a pile and forgot about it for a while. Later, after having shuffled things around, I don't remember which physical copy of the game is the one that is activated in my Steam account, and which copy is the extra one. Is there a way to find out what Key Code I used to add this game to my account so that I may determine which copy is still not yet activated?

[Hodman]

Do you get the same message after trying each code?

[MONROE]

right click the game in your steam library, manage- view CD KEY

[Shamanos]

Hodman - Yes.

Yawn - When I right click the game title, there are several View options (News, Forums, etc.) but no Manage option. I even looked in Properties and its various tabs. I checked my installed games and they don't have a "Manage - view CD KEY" either.