Encourage your candidate to ask others for 360º feedback regarding his planning and organizing skills. What are his strengths and what could be improved? Discuss the results with the candidate.
Investigate with your candidate what he has to learn in terms of planning and organizing his own work and, possibly, that of others.
Encourage your candidate to plan a large task (or several tasks) and ask him to explain how this task will be performed. Discuss the planning with the candidate. Ask for priorities, time management, costs, sub-tasks that will be delegated and so on.
Ask your candidate to define long term goals. Ask him to indicate what the priorities are and to make a planning based on those priorities.
Ask your candidate to check whether his time schedules are accurate. When did activities take longer than planned and why was that the case?