This paper aims to introduce Wizard of Oz methodology as an effective method in testing a serious game design which is aimed at enhancing collaborative learning on tabletops. The interface design enables mixed reality over tabletops to engage the players effectively in the learning activities. Additionally, the serious game scenario has been created to learn traffic rules and signs inside a city. The Wizard of Oz methodology was employed to experiment the design. Both questionnaire and observation were adopted to measure three perspectives of the serious game: supporting collaboration, facilitation of learning, and validation of design. The results of experiment by Wizard of Oz show that this method is highly effective in measuring collaborative learning in the serious game design. Moreover, the qualities of interface design and game scenario are explored during the experiment.