How to diagnose Overweight and Obesity ? 

The most common way to find out whether you're overweight or obese is to figure out your body mass index (BMI). BMI is an estimate of body fat, and it's a good gauge of your risk for diseases that occur with more body fat.
                
BMI is calculated from your height and weight. You can use the chart below or the National Heart, Lung, and Blood Institute's (NHLBI's) online BMI calculator to figure out your BMI. Or, you health care provider can measure your BMI.
                
Body Mass Index for Adults
                
Use this table to learn your BMI. First, find your height on the far left column. Next, move across the row to find your weight. Weight is measured with underwear but no shoes.
                
Once you've found your weight, move to the very top of that column. This number is your BMI.
                
This table offers a sample of BMI measurements. If you don't see your height and/or weight listed on this table, go the NHLBI's complete Body Mass Index Table.
                
What Does Body Mass Index Mean? 
                
Although BMI can be used for most men and women, it does have some limits. It may overestimate body fat in athletes and others who have a muscular build. BMI also may underestimate body fat in older people and others who have lost muscle.
                
Body Mass Index for Children and Teens
                
Overweight are obesity are defined differently for children and teens than for adults. Children are still growing, and boys and girls mature at different rates.
                
BMIs for children and teens compare their heights and weights against growth charts that take age and sex into account. This is called BMI-for-age percentile. A child or teen's BMI-for-age percentile shows how his or her BMI compares with other boys and girls of the same age.
                
For more information about BMI-for-age and growth charts for children, go to the Centers for Disease Control and Prevention's BMI-for-age calculator.
                
What Does the BMI-for-Age Percentile Mean?
                
Waist Circumference
                
Health care professionals also may take your waist measurement. This helps screen for the possible health risks related to overweight and obesity in adults.
                
If you have abdominal obesity and most of your fat is around your waist rather than at your hips, you're at increased risk for coronary heart disease and type 2 diabetes. The risk goes up with a waist size that's greater than 35 inches for women or greater than 40inches for men.
                
You also can measure your waist size. To do so correctly, stand and place a tape measure around your middle, just above your hipbones. Measure your waist just after you breathe out.
                
Specialists Involved
                
A primary care doctor (or pediatrician for children and teens) will assess your BMI, waist measurement, and overall health risk. If you're overweight or obese, or if you have a large waist size, your doctor should explain the health risks and find out whether you're interested and willing to lose weight.
                
If you are, you and your doctor can work together to create a treatment plan. The plan may include weight-loss goals and treatment options that are realistic for you.
                
Your doctor may send you to other health care specialists if you need expert care. These specialists may include:
                
An endocrinologist if you need to be treated for type 2 diabetes or a hormone problem, such as an underactive thyroid.
                
A registered dietitian or nutritionist to work with you on ways to change your eating habits.
                
An exercise physiologist or trainer to figure out your level of fitness and show you how to do physical activities suitable for you.
                
A bariatric surgeon if weight-loss surgery is an option for you.
                
A psychiatrist, psychologist, or clinical social worker to help treat depression or stress.