Finding a specific 'Cas cheats on Dean' fanfic can be a bit tricky as it's not a very positive or common trope. You can start by looking at the tags on fanfic websites. Tags related to Castiel, Dean, and relationship drama might lead you to such stories. Also, be prepared for a variety of interpretations and writing styles in these fics.
There are many fanfic communities out there. Some might have 'Cas cheats on Dean' stories. You could check out Supernatural - related forums where fans discuss and share their favorite fics. But keep in mind that the idea of cheating might not be everyone's cup of tea in fanfiction either.
Chapter 501, "This Night, Destined to be Turbulent", is the clearest early answer because it turns Dragon Ridge Island's situation into a concrete plot event. The chapter turns that into something concrete by moving from After settling in, Jiang Mingyu aimlessly strolled the streets.
Chapter 577, "Cat and Mouse Game", is the clearest early answer because it turns Cat's situation into a concrete plot event. Early on, Chen Manman also agreed with what Luo Xuan said.. By the end, What is this?" Guo Xing looked at the odd name..
Chapter 580, "Old Sir Gu: I command you to start a live stream!", is the clearest early answer because it turns Zhou Xu's situation into a concrete plot event.
Chapter 512, "The Ultimate Rich Foolish Uncle", is the clearest early answer because it turns Ling Bei's situation into a concrete plot event. Early on, An Jing would have been surprised if she could recognize him; in her.... By the end, I meant he's just unlucky;
Chapter 256, "Bonfire Feast", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. The chapter gives readers a real answer by opening on Turning around, Jiang Yan and Zhao Lang found their heads had been...
Chapter 703, "Slap Oneself", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with Wu Daoshan quietly raised his head to size up Zhao Lang, and sure... and closes on Young Master Gu was intending to back Jiang Li..
Chapter 442, "Let's Form an Alliance", is the clearest early answer because a shaky connection becomes usable in the plot. A useful checkpoint is the way the scene starts with Chang Rong was bewildered, "So, she seems pretty impressive." More...
Chapter 332, "Start the Live Stream!", is the clearest early answer because it turns Jiang Shixu's situation into a concrete plot event. The chapter gives readers a real answer by opening on His hands shaking, Jiang Shixu reopened the video.
Chapter 337, "Over Two Months Pregnant", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with He Hui's eyes widened with shock. and follows through with Tang Jinyu, what do you mean?!.
Chapter 520, "Was Zheng Xun Replaced?!", is the clearest early answer because it turns Zheng Xun's situation into a concrete plot event. The chapter gives readers a real answer by opening on What did it mean that Jin Siyue had already given him all the shares?!