How to see activation key on installed Steam games?

unfortunately you really can't, steam doesn't go and show you what key you've used to add a game. The best you can do if it's only a case of two keys is hope for the best and if the person comes back saying the key doesn't work you can apologize and provide the other.

Restart steam and Click forgot username.

Select "I remember code used"
Enter code
If you receive forgotten username email, it's code you used.

Thank you for your replies. Finally I tried to activate the codes I thought were already activated and confirmed it was like that. I dunno what could have happened if they were the not registered ones :S

I don't know why Steam doesn't allow us to see the keys of installed games, If you can't use them again, what's the point of not showing them?

If you have Steam up and running, you can sometimes check the CD key of a game.

Library > Game > Show CD Key (right-click menu)

Also, you can check your purchase history on GMG. If you can't tell by the date you bought the game, GMG Support has the ability to send you a list of keys for any particular purchase. Of course, if you just activate your keys willy-nilly, you won't know which purchase you activated. P