Hi Cat2
I would be looking at running the Sales [Item Detail] report (Reports>>Sales>>Item) for that item for an extended period i.e. 01/01/1900 to 31/12/9998. You'll also have to ensure that the Sale Status is set to be All Sales - this way you are including orders and quotes. The same can be run on the Purchases side with the Purchases [Item Detail] report - Reports>>Purchases>>Item. These reports would need to be clear i.e. show no transactions in order to move that item to be inventoried.
It should be noted that some customers rename the non-inventoried item and make it as inactive then create a new item that is inventoried, rather than deleting and recreating transactions. This could be an option for yourself in that position.