I have a steam game key, but I don't know which game is it, is there a way to find out?

If you install SteamDB browser extension and try to activate your key at https://store.steampowered.com/account/registerkey it will show you what the key is for provided the key is for something already activated on this account.
If you don't have the item already there is no way to find out before activation i.e. your key is consumed in the process. If you have a Steam friend with a huge library and you can trust her/him you could ask to try the key for you as they already have what the key is for probably.