Here are the 5 questions that I will ask:
- What is Box Model in CSS? - Every CSS developer should know the basics of box model.
- What are CSS Flexboxes? - I think this is the future of CSS. This will be the best way to structure future web apps.
- What are Sass, LESS, and Stylus? - They should have the basic knowledge about the CSS pre-processors.
- How floats work in CSS? - Many novice web developers fall prey to this concept.
- What are your favorite web design workflow tools? - They should be familiar with most popular tools for web designers.
IMO, this should give a clear idea about the CSS developer that I am going to hire. :) Best wishes.