Currently, e-learning systems are mainly web-based applications and tackle a wide range of users all over the world. Fitting learners' needs is considered as a key issue to guaranty the success of these systems. Many researches work on providing adaptive systems. Nevertheless, evaluation of the adaptivity is still in an exploratory phase. Adaptation methods are a basic factor to guaranty an effective adaptation. This issue is referred as meta-adaptation in numerous researches. In our research on the development of an evaluation framework of adaptive web-based learning systems, adaptation method assessment is a fundamental aspect. Currently, measures significantly lack to express the adaptive systems features and need to be explored. Consequently, we propose a three-fold approach. Firstly, specific adaptation measurement criteria are suggested. Secondly, experts and learners assess these criteria and both current learning situation and similar past experiences are considered. Finally, fuzzy group decision making theory is adopted to integrate different perceptions related to the adaptive system.