Clear verbal communication is a key skill for the modern scientist. In this article we describe two model science courses, one given to a small class and the other to a large class, in which verbal communication is emphasized as a key learning objective. Both courses use a form of problem-based learning (PBL) with an end point that involves students communicating their results and ideas verbally. Hence, assessment of verbal communication is necessary. We describe several different approaches to this challenge and highlight their applicability to small and large class sizes. We then show that, with careful application of these methods, a good relationship can be obtained between scores on verbal assessment exercises and scores on written examinations. On this basis, we argue that the verbal assessment can give an accurate reflection of the student's ability and can therefore be used as a method of formative evaluation in addition to being a component of the overall grade. (Contains 1 table and 2 figures.)