To tailor instruction to levels of learner proficiency in a domain in adaptive learning environments, rapid on-line methods of assessing learner knowledge and skills are required. Cognitive studies indicate that major factors in expert performance are organised structures in the person's knowledge base. Measuring different levels of acquisition of such knowledge structures should be a major purpose of cognitive diagnostic assessment. Traditional tests may not provide reliable evidence for diagnostic purposes and are not suitable for dynamic learning environments. This paper discusses an alternative diagnostic approach based on monitoring immediate traces of students' knowledge structures in working memory. Results of an experimental study applying this method to assessment of user reading skills are described. The study demonstrated a sufficiently high level of correlation (0.66) between obtained measures and traditional test scores, and a substantial increase in reliability, with testing time reduced by a factor of 3.8. (Contains 1 table and 1 figure.)