In the Search box in Keys in Preferences it should be possible to press the key and see what it’s already binded to; if that makes sense.
That’s a good idea. I would amend that suggestion by being able to also search by key/combo. That way a user can still use the key shortcuts to control Renoise even when the preference window is open.
Hack: Go to the key assignment thing, pick something off the list, and try to assign a new key combo. If that combo is already in use, it will tell you what it is assigned to.