In a user-centered design process, graphical user interface (GUI) prototypes may be seen as an important early artifact to design and validate user requirements before making strong commitments with a full-fledged version of the user interface. Ensuring the consistency of GUI prototypes with other representations of the user requirements is then a critical aspect of the design process. This paper presents an approach which extends Behavior-Driven Development (BDD) by employing an ontology in order to provide automated assessment for GUI prototypes as design artifacts. The approach has been evaluated by exploiting user requirements described by a group of experts in the flight tickets e-commerce domain. Such requirements gave rise to a set of User Stories that have been used to automatically check the consistency of Balsamiq prototypes which were reengineered from an existing web system for booking business trips. The results have shown our approach was able to identify different types of inconsistencies in the set of analyzed artifacts, allowing to build an effective correspondence between user requirements and their representation in GUI prototypes.