It was mostly Old Man Mu's actions that gave the two of them away.
Old Man Mu loved to prattle, and about 80 percent of his words were exaggerations. Therefore, when he described his 'fellow male junior apprentice' in a vividly weird way, Gu Shenwei was not suspicious, thinking of that as normal of the old man, but it was something Shangguan Fei said that alerted him.
Shangguan Fei had asked Old Man Mu, "If Taoist Yu is your fellow disciple, why did he save me not you?"
Old Man Mu was thenangry from embarassment but this too was normal of him; he was rarely kind to Shangguan Fei.
Gu Shenwei immediately grew suspicious and tried looking at the entire affair from Old Man Mu's perspective. That was, the Old Man Mu of the past, the one who had peerless kung fu skills, loved to kill and was bent on achieving complete freedom.