I was just looking at this rule: https://en.wikipedia.org/wiki/Y%C7%92u_bi%C4%81n_d%C3%BA_bi%C4%81n

Usually you'd rely on educated guesswork like this - and in many cases the character isn't pronounced exactly the same because of drift (https://en.wikipedia.org/wiki/Chinese_character_classification#Sound_change), but Chinese isn't as precise as many people make it out to be: "When one encounters such a two-part character and does not know its exact pronunciation, one may take one of the parts as the phonetic indicator. For example, reading 詣 (pinyin: yì) as zhǐ because its "side" 旨 is pronounced as such. Some of this kind of "folk reading" have become acceptable over time – listed in dictionaries as alternative pronunciations, or simply become the common reading. For example, people read the character 町 ting in 西門町 (Ximending) as if it were 丁 ding".