Generally interested to hear what questions, tests, etc. you have for designers, and why? Could be interesting to hear the differences (or similarities) between organizations, countries, type of designer or level.
If you need to hire a designer you need to make a complete technical task for the test. You must specify what should be shown, the theme, the style of the image, the style of writing the text, etc. You need to provide a technical task to the designer, to indicate the terms of implementation. After receiving the sketch, you should evaluate it, ask for corrections and decide whether the given designer is right for you or not.