Mostly in our conversations, in an informal way. I would say the assumption we make is that developers with a penchant for any form of functional prog and/or Lisps, and with an acute focus on testing have an easy time grasping Clojure. And we bias our rubric towards this.

Also, we encourage our candidates to choose Clojure for their take-home assignment, even if they do not have any prior taste of it. This, for those of them who choose that option, gives an idea of if they would enjoy working in it. Our rubric however does not have a bias against developers who do not choose Clojure for their assignments, and we make this explicit to the candidates as well.