Great question! Cognitive activation seems to be key for learning, but how can one measure mental processes and not use the products. From the literature I know, I see three options:

1. define and observe valid behavioral criteria, based on empirical research, such as they are listed in the Orientierungsrahmen Unterrichtsqualität, section 10, or use observed time-on-Task (Helmke & Renkl, 1992, 1993; Münchner Aufmerksamkeitsinventar)

2. use Student self-report of mental activity, as in the Paas-Scale or in Krell's questionnaire: 

Krell, M. (2015). Evaluating an instrument to measure mental load and mental effort using Item Response Theory. Research Letters, 1-6.

Paas, Fred GWC, and Jeroen JG Van Merriënboer. "The efficiency of instructional conditions: An approach to combine mental effort and performance measures." Human Factors: The Journal of the Human Factors and Ergonomics Society 35.4 (1993): 737-743.

3. use think-aloud techniques to describe degrees of complexity in student explanations (see attached papers).

Let me know how you did it and what your experience is.

Thanks again for the question,

Margarete Imhof