A way to recognize a Steam game by only having the key?

I believe ASF can do this. ask archie or search the ASF thread

The technology isn't there yet.

Only if you own the games involved...

If you own both games, then when you try to redeem it, it may tell you which game it is (and that you already own it). But if you don't own both games, then you run the risk of redeeming it to your account.

I have seen some instances where if you try to redeem a key for a game you already own, it will tell you you own it, but it will not tell you which game it is.

Another caveat is that if you own the game in question, but the key is for a game+DLC that you don't own, then it will still redeem to your account. So if you don't know exactly what the key unlocks, you could accidentally redeem it even if you own the base game.

Steam is not GOG

Most likely it's this one.
http://store.steampowered.com/app/315550/Taxi/

Do you remember what bundle or shop you got it from?

So my question is the same one as in title: Is there a way to find out what game do you have if you only have a key for it?

If you own the game already, ASF can tell you exact packageID when redeeming. If you don't, you'll get both this information, as well as +1 game on your account.

Even if you don't, you can always find somebody that own both and ask him kindly to check that, assuming you trust the person to not run with the key that is.

There is no other way to extract that info.

Well the only way is to try to activate it. If you own it and click next , it SHOULD prompt you to the installation window and it will say which game you are trying to install.
If you don't have it and we'll +1 to your library I guess.