How can I tell whether it's "can't" or "won't"?

Because "can't (a skill issue)" and "won't (a motivation or situation issue)" need different responses, telling them apart helps. A clue: can they do it on another day, in another situation, or with adult help? If they can with help, or it changes with mood or partner, it may be a motivation, anxiety, or predictability issue. If it's hard under any condition, treat it as "still difficult" and it's more effective to break down the steps or assist. Before deciding "they're just unmotivated," checking for difficulty, anxiety, or things not getting through reveals the support needed.