Question about steam keys

You can't know, it will tell you that you own the game no matter if the key is used or not

There is no way to tell whether the key has been used or not.

I meant, if i use a key for a game that i already own and then i give that key to a friend, can he still activate it?

I'm pretty sure that the key is still valid. There's no reason why it should be invalidated.

But if you want to be sure, just try to use the key a second time. Because on second time, if it was invalidated, it should say something like "invalid key" instead of "product already owned", am I right?

I enjoy Sour Cream with my baked potato, thanks :)

EDIT: Seems I was not right! I just tried with a key I used yesterday myself and Steam will only say "product already owned" without even checking if the key is still valid or not. So, what TempeteJoachim and Freiwilliger have said applies (I didn't mean to disagree, their posts were just faster): You can't tell.

Keep in mind if you want to test keys that way, some will register on your account even if you already have the game (but it will say then activation successful).

Alright everyone. Thanks for the fast answers :D
Here's your potato. Take it while you can :D
(i lied. still dunno how it works =) )
EDIT: Nevermind :D

View attached image.