I hope this wouldn't be the only way. I was thinking MC would tinker on Hana's phone again, find the video that she sent where they did it raw and where the MC thinks that the video might be old, check the date of the recording and voila, it is indeed old. Then he also finds the "cut" video from earlier, or maybe the raw sex video "was" the cut video in the first place! And then he finds more. Like when she's sucking the snake venom, she was really giving a bj.
Then instead of MC saying it directly that he found out she's lying, he asks questions instead, maybe during sex or teasing: "why did you cut the video, "is the raw video you sent me perhaps "old", "when you suck the venom out of the boy, how's the texture" etc.. Hana took notice and either admits to it or tease him to let his imagination run wild: "oh the venom was thick and white. A really "big snake" was also nearby at that time and I think it would be dangerous so I "took care of it".
Ah shit my imagination is running wild.