I would like to remove a mode that I added. Is there a way I can check where it is in use? If I delete it and it is in use, will it be deleted or will I get a message that it is in use?
You won't get a message, and there is no way to know where it's in use. Suppose it's in use in a Restriction "Only when mode is x", after removing it that mode would never show up so the restriction wouldn't happen. Were an app like Mode Manager or RM to try to set the mode to the missing mode, I think that would throw an error.