A 67-year-old man had a physical examination prior to beginning volunteer work at the hospital. A routine chest x-ray demonstrated left ventricular hypertrophy. His blood pressure was 180/110. He is 45 lb overweight. His diet is high in sodium and fat. He has a strong family history of hypertension. The client is placed on antihypertensive medication; a low-sodium, low-fat diet; and an exercise regimen. On his next visit, compliance would best be determined by: