Google created material design as a visual language that synthesizes classic principles of good design with the innovation and possibility of technology and science. This is a more specific than generic question, which should also be answered by a more specific answer.

Professional UI designers keeping their fingers on the pulse of the latest user interface updates and standards, will know a lot about material design: it's principles, properties, environment, goals, and the like. Hearing the designer's own thoughts should indicate that the candidate is not just following latest trends because it's trendy.