Identifying the study subject, "to be in meridian or collateral", is the first step of meridian-collateral differentiation. Combined with the ancient literature and clinical practice of acupuncture and moxibustion, the methods of identifying "to be in meridian or collateral" and its values are elaborated. Through analyzing the onset location of symptoms and the body layers of lesions, as well as the palpation and observation in meridian-collateral detection, "to be meridian" or "to be collateral" is identified in disease diagnosis. It may guide the choice of techniques of acupuncture and moxibustion. Identifying "to be meridian or collateral" is crucial in meridian-collateral differentiation, which not only contributes to improving the therapeutic effect of acupuncture and moxibustion, but also to better recognizing diseases and understanding the relevant concepts of techniques of acupuncture and moxibustion in view of acupuncture and moxibustion, and perfecting the system of meridian-collateral differentiation.