Being very clear what one means is the first assessment. Are you clear? Is the person capable of ASSESSING THEIR OWN UNDERSTANDING to have a dialogue until complete understanding. Basically, being able to explain what someone else is saying/meaning to a degree all parties are confident the message was received exactly as it should be--the same---exactly the same. Congruent [in math].
OR
"Are you picking up what I'm putting down," in the most literal way there is. That's Qs and As in education.