Montessori designed her materials so that childrem can not only understand the numbers as part of a greater system, they could feel the weight of each number in comparison to the last, they could see how big they were and comprehend that all numbers are linked and successive.
This is how we assess in Montessori classrooms. No need for formal assessments here. This guy knows his place value number system and there's no doubt about it.
That's how we