After Xu Da and Chu Nan found out that Ning Cheng's true identity was very likely to be a police officer, they knew for sure his information wouldn't be retrieved that easily. As Xu Da and Chu Nan were no longer working as officers, moreover, even if they were in the police station, the identity of an undercover police officer was top secret.
However, they managed to deduce the year Ning Cheng entered the police academy via his age. Then, they went to the police academy to find a person with connections and found the teacher that Ning Cheng had studied at the police academy back then.
That teacher was retired for a few years. Now an old man with a full head of white hair, a hint of regret appeared in his eyes when he saw Ning Cheng's photograph.