“How do you diagnose a sleeping disorder?”

The best way is to seek a referral and a consultation at a sleep medicine clinic, for example the Cornell Weill center for sleep medicine in New York City. If on the basis of structured interviews you meet criteria, you will be asked to stay overnight in a special room attached to various monitors assessing your heart function, breathing, oxygen saturation, breathing interruptions (apneas) and other important information. This will provide the information necessary to make a definitive diagnosis, which will guide the course of treatment. One example is a CPAP machine, for Continuous Positive Airway Pressure. This directs a stream of air through a mask into your nose and mouth which keeps your airway from closing up during sleep. Such closings can cause multiple interruptions in breathing per minute and severely disrupt sleep.

Pretty much like every other medical disorder: have a proper evaluation by a physician. A sleep study is not always indicated. You should start with your primary care physician and then appropriate referral as he deems necessary.

He needs a sleep study called a polysomnogram that measures breathing and brain waves and oxygen and pulse or EKG and breathing muscles and possibly carbon dioxide.With that information an exact diagnosis can be made. A home system with somewhat simpler monitoring may also be appropriate. Talk to your pediatrician about a referral to a sleep doctor who can do this study.

Take a peak at this link...
https://www.healthline.com/health/sleep/disorders