Or, on an object-by-object basis, open each object and the Attributes tab, and in the Attribute Display field select Custom Defined Attributes. The resulting filter will have some attributes that have been added via CA add-ins (like the PMO Accelerator), but if your developers have been following best practices, they will have used a consistent prefix for your environment in the attribute ID field, so it should be easy to see the ones specifically created for your system.
You may also have some custom Objects, and in the Object List View, those will all have a box at the left side of the row that would allow you to deactivate or delete them. You can only deactivate or delete custom objects.
John