In a report you can see the custom forms attached to the object (and click them to edit) using this text mode:
For any object with Custom Forms:
Edit Custom Forms in a pop-up window:
displayname=Click to Edit
As you can see from the tile name the collection on an object is called objectCategories or OBJCAT and is a referenceable collection and not a string as we find in some objects. While I haven't tested the API call for this it should be similar to referencing other collections.
You do need to search for the projects that have this in the objectCategories as there is no link from the custom forms to the projects, just the other direction.