Learning Games (LGs) are educational environments based on a playful approach to learning. Their use has proven to be promising in many domains, but is at present restricted by the time consuming and costly nature of the developing process. In this paper, we propose a set of quality indicators that can help the conception team to evaluate the quality of their LG during the designing process, and before it is developed. By doing so, the designers can identify and repair problems in the early phases of the conception and therefore reduce the alteration phases, that occur after testing the LG's prototype. These quality indicators have been validated by 6 LG experts that used them to assess the quality of 24 LGs in the process of being designed. They have also proven to be useful as design guidelines for novice LG designers.