If you get the Unreal source, its under Engine\Source\Programs\SlateViewer. It calls a test suite (RestoreSlateTestSuite()) which is in Engine/Source/Runtime/Slate/Private/Testing/STestSuite.cpp.
If you compile the code, you can run the SlateViewer app. It is a standalone slate app. It gives an overview of all widget possibilities. However it asserts on some operations.
Personally, I am having trouble wrapping my head around how Slate works, especially creating in-game UIs with it. How to create menus, delegates, how to invoke it (from HUD class? from elsewhere?). Docs say “examples abound”, and they do, but are really hard to figure out. Well, for me anyway 
We need code samples for menus, buttons, delegates. Bleakwise’s tut is a start, but much more needed.
A visual editor would be a godsend, so I can actually code a game instead of learning Slate’s dirty tricks 