Lilian Cheung, D.Sc., R.D., director of health promotion and communication, Department of Nutrition, Harvard School of Public Health, and coauthor of Be Healthy! It’s a Girl Thing: Food, Fitness and Feeling Great! (Crown, $13)

Anyone can write a diet book — what are the author’s qualifications? Is he or she a registered dietitian? An M.D. with training and experience in nutrition? A nutritionist with a degree from a credible scientific establishment, such as a university, as opposed to a degree obtained by mail?

See how many studies the diet is based on. If just one, how large was it?

Any scientific evidence presented in the book is important. Anecdotal information, such as case histories, is interesting but is not hard evidence of the diet’s effectiveness.

Pay attention to warnings and anything relating to risk factors you may have, such as diabetes, high blood pressure, or heart disease. Ask these questions: Is the diet suitable to your needs and lifestyle? Are there any health risks? Does the diet need medical supervision? If it recommends potentially dangerous herbs, food supplements or pharmacological agents, check with your doctor first.

Be skeptical of language that makes promises (as in “guaranteed to keep weight off”).

Consult other sources, such as the National Institute of Health’s Weight Control Information Network at www.niddk.nih.gov (type “win” in the search box), the Harvard School of Public Health’s www.hsph.harvard.edu/nutritionsource, and the FDA’s www.fda.gov.