Rapid cognitive diagnosis allows measuring current levels of learner domain-specific knowledge in online learning environments. Such measures are required for individualizing instructional support in real time, as students progress through a learning session. This article describes 2 experiments designed to validate a rapid online diagnostic method that was inspired by experimental procedures applied in classical cognitive studies of chess expertise. With the described rapid verification method, learners are required to rapidly verify suggested steps at various stages of a problem solution procedure. In this study involving 33 university students, a high degree of correlation was found between rapid testing scores and results of in-depth cognitive diagnosis based on observations of problem-solving steps using video recordings and concurrent verbal reports in the domains of kinematics (vector addition motion problems) and mathematics (transforming graphs of linear and quadratic functions). The article discusses possible applications of the suggested method in adaptive learning environments. (Contains 3 figures and 2 tables.)