There are two methods.
The first method is to listen to the five elements. This was in the early development of the Bazi theory, mainly during the Tang Dynasty (the representative master of this period was Li Zhongxu, whose masterpiece was "Li Zhongxu's Ming Shu", and the great writer Han Yu once wrote an epitaph for Li Zhongxu). During this period, it can be seen that What a person's horoscope and five elements belong to are generally judged based on the year of birth. The year is used as the reference point or reference coordinate in fortune-telling, and there are formulas to judge the five elements.
The second method is to use the day stem (the heavenly stem on the day of birth) to discuss a person's five elements. This appeared after Xu Ziping, a master of the Song Dynasty (because of him, eight-character fortune telling is also called Ziping) developed the eight-character theory. From then until now, in fortune-telling, the day stem represents the life owner himself, and the day stem is used to represent the owner of the life. As a reference point or coordinate system for fortune-telling.