Continuity mistake: When Liv says to Chad, "Aw, you gonna stalk me like a jealous boyfriend?" outside the school, her head goes from tilted 90 degrees to the left to suddenly being tilted about 45 degrees to the right between cuts.
Continuity mistake: When Chad and Liv are talking about how he sent her a "Find My Fam" request outside the school, Chad starts to turn his head from Liv and towards Wes and Amber. Then, it cuts to a close-up of him, and he is suddenly looking at Liv again and turns his head towards Wes and Amber a second time.
Continuity mistake: When Sam and Ritchie are outside the bowling alley and Sam says, "I gotta go!" after getting the call from Wes, suddenly she has her hands up by her face instantly between cuts, right before Ritchie stops her. (It happens quickly, and she's facing away from the camera, so it can be hard to catch.)
Continuity mistake: When Sam's phone rings outside the bowling alley, and she pulls it out, the way she's holding it changes completely between shots.
Continuity mistake: After Ritchie and Sam kiss outside the bowling alley, watch closely. Sam goes from having her left arm down below frame to up touching her chin instantly between cuts.
Continuity mistake: When Ritchie approaches Sam outside the bowling alley and jokingly asks her what she just took, when the camera cuts from one angle to the next as Sam replies, "Your boner pills," the look on her face changes completely between cuts. Additionally, in the first shot, she was holding up an energy drink that should be visible in the second shot, but isn't.
Continuity mistake: In the opening scene, when Tara mentions how she saw "Stab" once when she was a kid and the caller says "You live in Woodsboro and you don't know 'Stab?!'", the way Tara is holding the phone changes slightly between edits.
Continuity mistake: In the opening scene, when Tara puts the mail down on the counter and stands next to it while on the phone, the position of her body and the mail changes slightly when it cuts from the medium shot to the wide shot.
Continuity mistake: When Tara is in the hospital and is about to (accidentally) hit Ritchie with the phone, the way she's holding the phone changes between cuts. In some shots, she's holding it right by her head; in others, she's holding it above her head.
Answer: As far as I can tell, It's never stated in the movie how he found out about that. Maybe Sam told him off screen when he "learned " that Billy Loomis was Sam's biological father.