In Journey to the West, when Wukong became Niu Wangmo and cheated Hong Haier, how did Hong Haier decide that this father was fake?

Niu Wangmo, who came from Wukong, said that he was a vegetarian and didn't eat meat, which aroused Hong Haier's suspicion.

Forty-two times in The Journey to the West, the Monkey King came to Huoyun Cave pretending to be Niu Wangmo. Hong Haier invited the Monkey King to eat Tangseng meat, while the Monkey King assumed that he would be a vegetarian. Hong Haier thinks that Niu Wangmo is a monster who eats people and won't be a vegetarian, so he suspects that Niu Mowang is fake.

On the 42nd trip to the West, the Great Sage visited the Guanyin Charity Association in the South China Sea and bound Hon Hai.

-Monkey said, "So Xianlang is resourceful, but it was his enemy who invited me to eat the meat of the Tang Priest. Why don't I eat today? " The demon king asked, "Why don't you eat?" "I'm getting old recently," Monkey said. "Your mother often advised me to do something good. I don't want to do anything quickly and well. " The demon king said, "I wonder if my father is a vegetarian or a vegetarian?" "It's not a long fast, nor is it a month fast," Monkey said. "Call Lei Kuai, there should be only four days in a month." The demon king asked, "What were those four days?" "Three emotions lead to the sixth day," Monkey said. Today is a sunny day, on the one hand, it is fasting, on the other hand, guests are not welcome. Wait until tomorrow, I will wash and steam it myself and share it with the children. " When the demon king heard this, he thought, "My father used to eat people for a living, but now he is over 1000 years old. How can he eat quickly? I think, if I have done all kinds of evil and fasted for three or four days, where will there be good deeds? This statement is false, suspicious, suspicious! "