Item Type Usage
From both the item type list and the individual item types you can click the Item type usage button to access an overview of where item types are used (Figure 1.1).
You can filter on item type, website, and also the kind of item (used in).
Please note, that this list will not inform you if an item types is used in another item type, whether through inheritance or through the Item item field.
Always back up your item types before deleting an item type - I speak from painful experience.