Regardless the large efforts done in assessing user experience in virtual reality games, few instruments have been proposed to conduct these evaluations on VR serious games. Most proposals focus on evaluating final products or address different subsets of design aspects, providing few or no feedback to designers during the development of a game. In order to help developers assess their prototypes and determine the soundness of their interim designs during the development of VR serious games, this article proposes a formative evaluation instrument based on a literature review. The instrument has been particularly designed to evaluate user experience and provide feedback to designers for improving their proposals when required. The usability and usefulness of this instrument was evaluated with experts in the design of serious games, and the preliminary results are highly positive.