The most obvious thing was the fact that the spy was too concerned about Yao Ling's condition. He did it in a subtle way by a few glances here and there or deliberately went closer to Xiu with a helpful smile, and under normal condition, they wouldn't be able to find out that something was wrong, but because they had already let up their guard, they were able to find the abnormality easily.
The spy kept going around Yao Ling's room a few times and yes, he was serving other people as well. But there was one thing that he couldn't cover up, his eyes couldn't lie. He always looked at Yao Ying and Yao Ling's room with a meaning glance in a flash, seemingly to wait for something exciting to happen.