Small question about the "STEAM keys"?!

Hi,
just have a few questions:

1. Do you know is there some some tool which can check if some key is active or not?

2. If you get some key, which is a DLC. How can you know "for what game is that key"? Or maybe what "DLC is the key for"?
   Recently won some game or DLC, turns out it's a DLC. Can't activate it, as that particular game is not within my small list of STEAM games.
   Is there a way to find for which game is it or what that DLC is from a key?

Thank you for answers,