A shortcoming of current evaluation methods of exploratory search engines is that they cannot be used to determine whether users' behaviours and exploratory search tasks are usefully supported. One main reason is that these methods rely on a loosely defined model of the exploratory search process. The aim of this paper is to propose a new model-based heuristic evaluation method of these systems. We present the approach used to design the heuristics of exploratory search and a checklist form for using them. The evaluation of the heuristics and its procedure demonstrates that they significantly help the users achieve an exploratory search system assessment.