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:
In "Journey to the West", Sun Wukong turned into the Bull Demon King and was discovered by Red Boy in the fortieth chapter.
The main story of Chapter 40: The red boy used the wind to frighten Tang Seng away. Wukong learned that Red Boy was the son of his elder brother, the Bull Demon King, whom he had sworn to five hundred years ago, and wanted to use this relationship to repay his master. Red Boy didn't believe what Wukong said, so he blew out fireworks from his mouth and nose to burn Wukong Bajie away. Brother Dragon King was invited to spray water on Red Boy, but Wukong, who was covered in fireworks, was almost killed.
Bajie went to invite Guanyin, but the monster pretended to be Guanyin and got into the cave. Wukong transformed into the Bull Demon King and was questioned by Red Boy. He somersaulted into the South China Sea and asked Guanyin to subdue Red Boy.
Baidu Encyclopedia—Journey to the West