You can check your Danjeon breathing for yourself as follows
 
Danjeon breathing must be natural. You must breathe in and out to your ability, in other words, as naturally as possible. Holding breathing unnaturally may cause some serious side effects. Trying to lengthen your breath unnaturally is also dangerous and can cause deficiency of oxygen supply to the brain. Control your breathing so that you don't feel dizzy after you practice Danjeon breathing.
 
When you do Danjeon breathing properly, your upper part of abdomen hardly moves but only the area below the navel moves up and down and the peak is on the area of Danjeon.

On the other hand, the crest is formed on the navel or above it when you don't do Danjeon breathing correctly. This type of breathing is not Danjeon breathing but abdominal breathing.
 
The next method of checking your breathing is whether energy is accumulated in your Danjeon. Energy-accumulation means that energy is drawn into the Danjeon and accumulated there by Danjeon breathing. If you have a weak Danjeon even though you practice Danjeon breathing for a long time, then you must have done wrong Danjeon breathing. If you have done so, some warm energy goes around your whole body and your face turns red. That is a kind of side effect caused by improper breathing method.
 
Danjeon, the existence of energy, cannot be formed only by physical muscular movements of abdomen. When mind control is accompanied, you can do even more powerful breathing. You must learn how to control your mind, as well as how to do Danjeon breathing. Danjeon breathing practice may be said to be a process of controlling your undesirable emotions, such as the desires to meddle in other people's affairs, getting irritated easily, getting angry easily, or getting arrogant or haughty.
 
Danjeon breathing practice leads trainees to fall their brain waves into the band of alpha waves in the end. When your brain wave becomes alpha wave, a new stage of your practice is opened. If you lower the frequency of your brain wave through proper Danjeon breathing, you will be able to get into the world of alpha waves and to appreciate the real points of Danjeon breathing. As a rule, when you lengthen your breathing to one minute and get the synergism of your mindfulness and breathing, you will be able to get into the world of alpha waves.