The set of objects in a ULC application is defined in that application's context. When testing a ULC part from the Composition Editor, you can inspect the active context from the System Transcript window. From the menu bar, select ULC->Debug->Inspect Builder Context. The context of the ULC examples is also accessible by selecting ULC and then Debug.
As soon as you are inspecting the context, you can send messages like findAllWidgetsOfType: 'Table' or findAllWidgetsNamed: 'MyWidgetName'. These methods are implemented for debugging only and are not part of the ULC API.