Sparring

"Oh, right? Did Master teach you the Heavenly Severing Sword? Yun Haoran pointed out the spear Yong Yunzi was holding, his eyes glimmered with curiosity. "I felt the essence of it in that spear." 

"No, Master showed it to me once." Yong Yunzi replied while acting she was thinking. "I thought that Sword was not fit for me so I practiced the Spear instead." 

Yun Haoran thought it was interesting. His suspicion of Yong Yunzi being a regressor is entirely a suspicion. If she has an unknown talent or she is that talented, it is given.

The moment she knew about the seed, he had his suspicions. It may be because Li Chao told him about it but it's entirely just a guess.

There is also evidence she was a regressor. It was about her Dao Heart, he didn't know if it was innate or acquired. His Dao Heart is innate and doing whatever he wants is the norm. He wanted to test it out himself.