The same way. KDE components are very modular, so it's trivial to write a UI form that displays a bunch of components from various parts of your code. You could write a second executable that just displays this test form with test data (most of your code should be written as a library anyway, so having two executables that call your components shouldn't be an issue).

Of course more automation is better, so I'd probably want to write a selenium-equivalent for asserting things about a component tree. Again, the KDE components are very regular so it's easy to generically traverse/inspect a GUI.

It's really not hard if you actually try.