Is there a way to know who redeemed which CD key or which game does the CD key belong to?

I bet most people are going to read the title of the thread and then vote no in the poll not knowing that the poll is a different question.

There is no reason why someone wouldn't want the ability to enter a key on Steam and be able to see information about it without activating it. I really don't see why Steam doesn't at least have the simple ability to enter a key and they tell you what game it is for and if it has been used or not.

Yes, there is a way, but not for the user. I can see why only providers can see who used a key, but I would like to identify a game from a key without having to activate it.

which game does the CD key belong to?

you can check this using Steam Database Add-on IF you already have that game on steam account by entering it on key activation page

To know which game does the CD key belong to, go here. with a script like this chrome extension installed for example, for easier and more convenient game identification.

If the CD key has been used, and your steam account does not own the game the CD key is attached to, then a messsage will appear saying that the CD key is duplicate meaning that it was already used. However, previously, if you redeemed the key on the Steam application key redeemer, it use to pop up a "retrieve account" button which would send an email to the account that is attached to the account the CD key was redeemed on, however it seems they have removed that button. Personally I'm not sure if they send an e-mail automatically if that occurs at this current time.

If you have an unused or suspect to be unused CD key and you want to know what it is without redeeming it, there is another more sneaky, but FLAWED method. You can give it to someone, preferably to someone you trust, who has a LOT of games on steam(or at least owns the games you think it might be) and doesn't mind using the key on his account to check what the key is, preferably with the chrome extension installed on the Steam key redeem website. It would be helpful if you had a clue on what the key redeems, so you could dig through this user's account beforehand, to check if he owns the game you suspect it to be. When he uses the key, he will redeem the key's game content if he does not own the game, and he could tell you, or check his gamelist(if hes not very honest, but that would require more work for you to check upon since he has a LOT of games) for a new game if the profile is public, that's the end of it. If he does own the game, he'll be able to tell you which game it is, and the key will continue to be unused(because you know or suspect that it is). However that still does not tell you if the key is still unredeemed, as someone could have used it by guessing the key by random, revoked by the developer, the user with a LOT of games being sneaky and stealing it and using it on an alternate account, you accidentally leaked it on your online stream, you forgot that you redeemed it yourself a long time ago on your current or old account and now forgot about it, or that your own friend or the Amazon delivery worker accidentally saw it but since he has a photographic memory, decided to use it without you knowing about it. But at least you know what the game is, if he's honest and tells you. After all of that roller-coaster, you could then try it out on your account or give it to someone and see if it actually works, without knowing if it really does or not. The reason why you'd pick someone with a LOT of games, or just someone with the games you think it could be, would be because that he's less likely to redeem/consume the key due his HUGE library of games.

Or, you could, you know, just try the key and see for yourself what it is, knowing that you'll redeem if the key is unused and if you don't own it.
If by some chance you do own it, you could try using an alternate account or give it to a friend without that game and redeem there to verify if the key was previously valid or not.

It's most likely, bordering on absolute certainty, intentional.

Even if there weren't concerns like the potential for abuse, you have to keep in mind Valve has no interest in making your 3rd party purchases, which don't net them a single cent, more convenient.
Afterall they want you to buy from Steam and not from Humble, Fanatical or even shady key reseller sites.

Yeah, I know: A 3rd party purchase might make Valve some money later down the road due to follow up purchases like micro-transactions or DLC but I don't think corperations go by that vague kind of rational.