Spoilers for Thousand Autumns Volumes 3 and 4!!!
I didn't pay attention to it the first time I read Thousand Autumns, but during my reread it's obvious that Xie Ling and A-Yan knew that Yan Wushi loved Shen Qiao from the start.
Art by @_Konia_; reposted with permission.
It's fairly obvious that Xie Ling is infatuated with Shen Qiao immediately because his gaze never leaves Shen Qiao and Xie Ling trusts him completely:
Yan Wushi only looked at him, his eyes filled entirely with Shen Qiao’s reflection. “I’ll be taking your pulse.” Shen Qiao lifted his wrist, and Yan Wushi didn’t respond to that either, letting Shen Qiao handle him as he wished. But his eyes remained fixed on Shen Qiao—whether Shen Qiao was hunched over or sitting upright, Yan Wushi’s gaze never left him. ...Just as Shen Qiao was furrowing his brows and pondering, Yan Wushi suddenly smiled at him. This smile was different from those faint half smiles from the past, and lacked any sense of sarcasm, ridicule, or unbridled arrogance. It was simply a smile, nothing more, as if it weren’t Shen Qiao in front of him, but a beautiful flower. (vol. 3, pg. 85-6)
A-Yan tells Shen Qiao, "...I understand why my previous self treated you the way he did. He’s a paranoid person at heart, someone who’d never trust anyone else. No matter how good you are, he’d always want to bring out the darkness hidden within you. But he doesn’t know that you are you. There may be hundreds of thousands of Chen Gongs in this world, but there is only one Shen Qiao." (vol. 3, pg. 135)
Yan Wushi notices that, "There was still a hint of remaining warmth within his chest, left there by Xie Ling and A-Yan, from their feelings whenever they thought about Shen Qiao. But at this moment, Yan Wushi forcefully wiped it away..." He doesn't consider the the thoughts and feelings from the other parts of his personality to be his own." (vol. 3, pg. 142)
There's also the scene where Xie Ling tries to kiss Shen Qiao:
...Yan Wushi abruptly seized Shen Qiao’s neck, then he threw himself closer and bit down on his lips! In pain, Shen Qiao snaked his arm around Yan Wushi’s neck and struck hard. The other man fell limply on top of him. It was finally quiet. Shen Qiao let out a sigh of relief. He picked up Yan Wushi’s wrist and felt it, then let out a gasp of surprise. If the man had just been in the throes of qi deviation earlier, then now, only a short time later, his pulse had completely calmed. In contrast, his life force even seemed to be thriving? (vol. 3, pg. 225)
When Yan Wushi is back to his normal personality, he commented on the unskilled kiss: "What, did Xie Ling even forget how to kiss someone after losing most of my memories? He was so impatient he even bit you?" (vol. 3, pg. 232) Yan Wushi knows immediately that Xie Ling has feelings for Shen Qiao, but he doesn't realize his own feelings yet.
Later, Yan Wushi admits to himself that Xie Ling and A-Yan's thoughts where his own as well, and that's what leads to the confession scene:
Yan Wushi smiled but didn’t speak. Before, he’d loathed “Xie Ling’s” influence, thinking that those thoughts weren’t his own. He’d tried to suppress that strange feeling numerous times, and he’d believed that the moment he repaired the flaw in the demonic core, that feeling would vanish with it. He hadn’t expected that Shen Qiao’s smile would reawaken everything. He was unwilling to admit that he, who’d looked down upon everyone in the world, would one day find that a name had wormed its way into his heart. Human hearts were filled with malice. Some people were traitors, turning their backs on all integrity; some were ingrates, repaying kindness with enmity. There were also those who’d abandon their spouses, who were willing to do anything for wealth and glory. Yan Wushi had seen many, and he’d thought nothing of them because he, too, was a selfish, callous human. He only categorized things based on whether they deserved his notice, and to him, there were no actions that fell beyond a line that could not be crossed. However, now Yan Wushi was forced to admit that Shen Qiao was unique and that he couldn’t change him. Though the world was vast, there was still only one Shen Qiao. “My venerable self suddenly thought of something amusing,” he said. “Would you like to listen?” “No,” said Shen Qiao. Yan Wushi turned a deaf ear and started talking anyway. “Once upon a time, there was a man who found a stone in a heap of gold and jewels.” Shen Qiao’s mouth twitched. Didn’t he just say that he didn’t want to listen? “But he couldn’t believe that it was only an ordinary stone. He thought that since it’d been piled together with the mass of treasures in that room, it must also be a treasure. So, he brought it everywhere with him, and even had it examined and polished by many. But every single person, without exception, told him that it was only an ordinary stone, that there was nothing special about it. Guess what happened in the end?” Shen Qiao’s face was both lost and bewildered. “In the end, he finally believed that it was indeed a worthless stone. But in his eyes, compared to the room full of gold and silver treasure, even if it was only a stone, it was still one of a kind—a stone out of a million.” Shen Qiao was silent. Why did this story sound so bizarre? It was indeed unusual to hear such a normal story from such an abnormal person. He couldn’t help but say, “Even thousands of gold pieces cannot purchase happiness. Some people care little for wealth—they only wish to seek out things that other people find worthless. In my opinion, that man already liked that stone more than the other jewels and treasures, but he was trapped by his preconceptions and unwilling to admit it.” Yan Wushi laughed. “That’s true, you’re very correct. Thousands of gold pieces cannot purchase happiness." There seemed to be a deep significance within these words. (vol. 4, pg. 58-60)
