I don't know of anything that specifically does that.
However, if you just want to know if a keybinding is taken (and what it's taken by), you can just open up your keybindings window and click the unused second binding for something, and hit the key(s) you want to check. If it is taken, you'll see a message at the bottom informing you that the key(s) were unbound from their previous function. You can then decide if that previous function was something you wanted, and if it is, go re-bind the keys to it. A bit more complicated than what you're looking for, but it'll do the job.
Another solution would be outside of the game, open "World of Warcraft/WTF/Account/<AccountName>/bindings-cache.wtf" in Notepad or another plain text editor, and search for the binding you're wondering about, or just scroll through the list. They're stored in plain text, like "bind N TOGGLETALENTS" and "bind SHIFT-M TOGGLEBATTLEFIELDMINIMAP".
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