Encourage your candidate to read various texts and to comment on his strengths and weaknesses.
If your candidate tends to be long-winded, ask him to write down a summary of his own text and discuss this summary.
Encourage your candidate to ask others for 360º feedback regarding his written communication skills. What are his strengths and what could be improved? Discuss the results with the candidate.
Review a report, a note or a letter your candidate has written and provide feedback on the aspects mentioned in the examples.