A useful distinction
A tester can appear to understand a prototype when the designer supplies instructions after every hesitation. Observing first makes misunderstandings visible. The point is to learn about the current design, not to grade the person playing it.
Put it into practice
Agree on a short test and ask the tester to describe what they expect. Record the point of confusion and the information visible then. Offer help when needed, marking that intervention in the notes so independent understanding is not confused with a prompted result.