You might be limiting yourself in respect to the type of instrument. You might consider taking the general area of comprehension and see what that tells you. For example, using the WISC or similar test can tell you the interactions between comprehension and other learning functioning.  From there you might find there is a problem with auditory comprehension or visual functioning, etc. . Otherwise, there are many valid and reliable tests available. I suggest going to some of the better publisher's sites, talk with colleagues, etc. You'll really open up more possibilities that way. I hope that helps.

Hi Marcel!

You ask if there are any valid approaches for the evaluation of a learners topic comprehension/understanding. 

If I understood well your question, I think that Piaget's theory is an approach that is relevant to see to what extent a certain learning is a deep, meaningful, and comprehensive learning. I think you can profit a lot if you read his book: To understand in to invent: The future of education. 

According to Piaget, a given learning (e.g., that the number of elements in a set, 10 buttons, for instance, remains constant regardless of their spatial arrangement  if the respective set) is only deep, meaningful and comprehensive when, among other things, it leads:

(1)  to a true understanding rather than memorizing of what is taught.  In the example of learning mentioned above, when a true understanding is the case, then the set can contain any number of elements (10, 20, 30, 40, ....). If memorizing is the case, then that knowledge/learning is only valid when the set has 10 elements, neither less nor more.

(2) to generalization and transfer, that is, non only that learning and knowledge includes sets with any number of elements, but it is also transferred to other domains (e.g., the quantity of water in a glass does not depends on the tallness or width of the container).

(3) a deep, meaningful, and comprehensive learning, just because it relies on understanding, not memorizing, it endures for long, if not forever, say, in normal circumstances.

(4) when a deep, meaningful, and comprehensive learning is the case it resists even when efforts are make to extinguish it . As a Piagetian child once said, "when I really know and understand a certain concept (e.g., number conservation, liquids conservation), I know and understand it forever" 

(5) When a deep, meaningful, and comprehensive learning is the case, it modifies the learner's previous cognitive structures. This means that a deep, meaningful, and comprehensive learning  does not amount to knowing, as it were, "more of the same", but knowing "better and better".

(6) For a deep, meaningful, and comprehensive learning  to be the case, the learner should not be confronted with learning situations which are much below or above his/her cognitive abilities to understand. When the former is the case, there is nothing to be learnt. When the latter is case, nothing can be learned. Because of this, for a deep, meaningful, and comprehensive learning to occur, teachers/professors have to master their area of expertise and be well versed in the  details of the learner's psychological development.

Of course, teaching/learning/education is costly, but it is far less costly than its alternative, ignorance. Note also that only (good) education can save societies, and even individuals from possible collapse, be it violent or gradual.

I hope I have got your question and that this helps.

Best  wishes

it gives rise to understanding  

A critical appraisal of the topic or a critical reflection, supported by references to the topic literature.

Engage students in a project that requires students to classify, compare, explain, evaluate, argue/defend, appraise, and synthesize. Use a well-developed rubric to evaluate student performance.