Hi, It doesn't sound simple.
I think you can try open the "480 Dimension Set Entry" table in the development environment directly. (This does not require a development license)
Dimension in NAV is saved in the form of Dimension Set ID. A dimension set ID indicates a combination.
You can search this table to see if your Dimension has been used.

As for changing Dimensions, there is no good way in NAV2018.
Only to revese documents, such as Dimenisons in Posted Sales invoice are wrong, this need to create Sales Credit Memo to cancel it first.
Hope this will help.
Thanks.
ZHU