1. In a kit you configured, rename qmake to qmak_ (in the file system).
2. Start Creator.
3. Go to "Tools" -> "Options..." -> "Build & Run" -> "Kits".
   You'll see that the kit is marked as invalid.
4. Start creating a project using "File" -> "New File or Project..." -> "Other Project" -> "Empty qmake Project".
5. Proceed to the "Kit Selection" page of the wizard.
   You'll see a list of kits. The invalid kit is checked and does not show a warning.
6. Uncheck the invalid kit.
7. Check the invalid kit.
   Now you will see warnings.

Creator should have shown these warnings right from the beginning.