A 22-year-old man is persuaded by his wife to come to you for a general checkup. She hints of concern about alcohol use. He admits to drinking on average three to four beers every night with more on the weekends. He used alcohol rarely until 2 years ago when his brother died. He has never had withdrawal symptoms after several days of abstinence. What would be a practical next step to take that might help you further evaluate the physical consequences of this patient's drinking?