First, Red Boy became suspicious after Sun Wukong said to let Tang Monk go. Then, Red Boy asked Sun Wukong why it was my birthday today and why I didn’t bring a gift. Sun Wukong said he was in a hurry and didn't take it with him. He would send it off another day, so he saw through it.
Red Boy asked the little demon when he met the Bull Demon King. The little demon said that he met him on the way. At this moment, Red Boy was still not sure whether the Bull Demon King was real or fake.
Later, Sun Wukong said the word 72 transformations. Didn’t his father know 72 transformations? In order to further confirm the authenticity of the Bull Demon King, Red Boy asked about his birth date, because the Bull Demon King usually talked about Red Boy's birth date, but now Sun Wukong revealed his secret.
Extended information:
Red Boy heard that eating Tang Monk’s meat can lead to immortality, so he pretended to be a child in distress, tied his hands and feet with hemp rope and hung himself from a tree to lure Tang Monk’s master and disciples to rescue him. , but when Sun Wukong saw through it, he directly used the strong wind to sweep Tang Seng away. Later, he fought with Sun Wukong and transformed into Guanyin to trick Zhu Bajie into capturing him.
Sun Wukong was unable to subdue him for a while. Even if he invited the Dragon King of the Four Seas to rain, it would be difficult to extinguish the Samadhi True Fire. Sun Wukong could only invite Guanyin Bodhisattva to subdue the Red Boy, and extinguished the Samadhi True Fire with the nectar from the Jade Pure Bottle. He sent his apprentice Mu Cha (Muzha) to borrow thirty-six Tiangang knives from his father, King Tuota, and hid them in the throne of the lotus platform to trap Red Boy.
Guanyin Bodhisattva put five golden hoops on Red Boy (one of the three golden hoops), and finally subdued Red Boy, and he followed Guanyin Bodhisattva and became a good fortune boy. It finally worked out.