Your teen's body is going through many changes during puberty, when weight gain is common and expected. If your teen is not gaining weight, you need to first think about where he/she is in the course of his/her puberty. If he/she has already started his/her physical development, but is not yet gaining weight, that can indicate a problem. However, if your teen is just a late bloomer and hasn't really started his/her puberty just yet, then it might not be realistic to expect some weight gain yet.
If you want a more formal, clinical answer, your teen's doctor can help you. There is a common body fat test, called the body mass index (BMI), which is often used to classify someone as underweight, overweight, or at an ideal weight. The BMI uses height and weight measurements to come up with a number that is supposed to indicate how much body fat a person has. However, there have been questions regarding how accurate the BMI number is and it becomes even more challenging trying to apply the BMI scale on growing children. Your doctor will likely use BMI, but only as one component when determining if your teen is underweight. Some issues your doctor will consider are how physically active your teen is, his/her eating habits, and his/her medical history. So you should take a close look at these same issues.
If your teen appears underweight to you and you see other troubling signs, such as diarrhea, lethargy, or a cough that lasts at least two weeks, then your teen might be suffering from an underlying health concern. However, most teens who seem underweight are likely just developing more slowly. Keep an eye on your teen's growth over a period of time to see if the weight is just growing more slowly than his/her height. Eventually, most underweight teens will catch up.