Is this key real?

I created a topic on steamtrades and traded BioShock Infinite. (The guy I traded with seems to be a good guy but forget his name :( )

I´m new to this trading system and dont much of it.
So my question now is, how can i know if this key is real and works? ( I dont want to use it myself of course ;) ),
because I want to trade it further.
What should I do?